mm. The edge was covered with branched villi, while in the centre of

each of the flattened surfaces there was a spot free from villi. On the surface adjoining the uterine wall was a darker area (_e_) formed of two layers of cells, which is interpreted by Reichert as the embryonic area, while the membrane forming the remainder of the ovum, including the branched villi, was stated by Reichert to be composed of a single row of epithelial cells.

Whether or no Reichert is correct in identifying his darker spot as the embryonic area, it is fairly certain from the later observations of Beigel and Löwe (No. 228), Ahlfeld (No. 227), and Kollmann (No. 234) on ova nearly as young as that of Reichert, that the wall of very young ova has a more complicated structure than Reichert is willing to admit. These authors do not however agree amongst themselves, but from Kollmann's description, which appears to me the most satisfactory, it is probable that it is composed of an outer epithelial layer, and an inner layer of connective tissue, and that the connective tissue extends at a very early period into the villi; so that the latter are not hollow, as Reichert supposed them to be.

[FIG. 164. THE HUMAN OVA DURING EARLY STAGES OF DEVELOPMENT. (From Quain's _Anatomy_.)

A. and B. Front and side view of an ovum figured by Reichert, supposed to be about thirteen days. _e._ embryonic area.

C. An ovum of about four or five weeks shewing the general structure of the ovum before the formation of the placenta. Part of the wall of the ovum is removed to shew the embryo _in situ_. (After Allen Thomson.)]

The villi, which at first leave the flattened poles free, seem soon to extend first over one of the flat sides, and finally over the whole ovum (fig. 164 C).

Unless the two-layered region of Reichert's ovum is the embryonic area, nothing which can clearly be identified as an embryo has been detected in these early ova. In an ovum described by Breus (No. 228), and in one described long ago by Wharton-Jones a mass found in the interior of the egg may perhaps be interpreted (His) as the remains of the yolk. It is, however, very probable that all the early ova so far discovered are more or less pathological.

The youngest ovum with a distinct embryo is one described by His (No. 232). This ovum, which is diagrammatically represented in fig. 168 in longitudinal section, had the form of an oval vesicle completely covered by villi, and about 8.5 mm. and 5.5 mm. in its two diameters, and flatter on one side than on the other. An embryo with a yolk-sack was attached to the inner side of the flatter wall of the vesicle by a stalk, which must be regarded as the allantoic stalk[97], and the embryo and yolk-sack filled up but a very small part of the whole cavity of the vesicle.

[97] Allen Thomson informs me that he is very confident that such a form of attachment between the hind end of the embryo and the wall of the vesicle, as that described and figured by His in this embryo, did not exist in any of the younger embryos examined by him.

The embryo, which was probably not quite normal (fig. 165 A), was very imperfectly developed; a medullary plate was hardly indicated, and, though the mesoblast was unsegmented, the head fold, separating the embryo from the yolk-sack (_um_), was already indicated. The amnion (_am_) was completely formed, and vitelline vessels had made their appearance.

[FIG. 165. THREE EARLY HUMAN EMBRYOS. (Copied from His.)

A. An early embryo described by His from the side. _am._ amnion; _um._ umbilical vesicle; _ch._ chorion, to which the embryo is attached by a stalk. B. Embryo described by Allen Thomson about 12-14 days. _um._ umbilical vesicle; _md._ medullary groove. C. Young embryo described by His. _um._ umbilical vesicle.]

Two embryos described by Allen Thomson (No. 239) are but slightly older than the above embryos of His. Both of them probably belong to the first fortnight of pregnancy. In both cases the embryo was more or less folded off from the yolk-sack, and in one of them the medullary groove was still widely open, except in the region of the neck (fig. 165 B). The allantoic stalk, if present, was not clearly made out, and the condition of the amnion was also not fully studied. The smaller of the two ova was just 6 mm. in its largest diameter, and was nearly completely covered with simple villi, more developed on one side than on the other.

In a somewhat later period, about the stage of a chick at the end of the second day, the medullary folds are completely closed, the region of the brain already marked, and the cranial flexure commencing. The mesoblast is divided up into numerous somites, and the mandibular and first two branchial arches are indicated. The embryo is still but incompletely folded off from the yolk-sack below.

In a still older stage the cranial flexure becomes still more pronounced, placing the mid-brain at the end of the long axis of the body. The body also begins to be ventrally curved (fig. 165 C).

Externally human embryos at this age are characterised by the small size of the anterior end of the head.

[FIG. 166. TWO VIEWS OF A HUMAN EMBRYO OF BETWEEN THE THIRD AND FOURTH WEEK.

A. Side view. (From Kölliker; after Allen Thomson.) _a._ amnion; _b._ umbilical vesicle; _c._ mandibular arch; _e._ hyoid arch; _f._ commencing anterior limb; _g._ primitive auditory vesicle; _h._ eye; _i._ heart. B. Dorsal view to shew the attachment of the dilated allantoic stalk to the chorion. (From a sketch by Allen Thomson.) _am._ amnion; _all._ allantois; _ys._ yolk-sack.]

The flexure goes on gradually increasing, and in the third week of pregnancy in embryos of about 4 mm. the limbs make their appearance. The embryo at this stage (fig. 166), which is about equivalent to that of a chick on the fourth day, resembles in almost every respect the normal embryos of the Amniota. The cranial flexure is as pronounced as usual, and the cerebral region has now fully the normal size. The whole body soon becomes flexed ventrally, and also somewhat spirally. The yolk-sack (_b_) forms a small spherical appendage with a long wide stalk, and the embryo (B) is attached by an allantoic stalk with a slight swelling (_all_), probably indicating the presence of a small hypoblastic diverticulum, to the inner face of the chorion.

A remarkable exception to the embryos generally observed is afforded by an embryo which has been described by Krause (No. 235). In this embryo, which probably belongs to the third week of pregnancy, the limbs were just commencing to be indicated, and the embryo was completely covered by an amnion, but instead of being attached to the chorion by an allantoic cord, it was quite free, and was provided with a small spherical sack-like allantois, very similar to that of a fourth-day chick, projected from its hind end.

[FIG. 167. FIGURES SHEWING THE EARLY CHANGES IN THE FORM OF THE HUMAN HEAD. (From Quain's _Anatomy_.)

A. Head of an embryo of about four weeks. (After Allen Thomson.) B. Head of an embryo of about six weeks. (After Ecker.) C. Head of an embryo of about nine weeks.

1. mandibular arch; 1´. persistent part of hyomandibular cleft; _a._ auditory vesicle.]

No details are given as to the structure of the chorion or the presence of villi upon it. The presence of such an allantois at this stage in a human embryo is so unlike what is usually found that Krause's statements have been received with considerable scepticism. His even holds that the embryo is a chick embryo, and not a human one; while Kölliker regards Krause's allantois as a pathological structure. The significance to be attached to this embryo is dealt with below.

A detailed history of the further development of the human embryo does not fall within the province of this work; while the later changes in the embryonic membranes have already been dealt with (pp. 244-248).

For the changes which take place on the formation of the face I may refer the reader to fig. 167.

The most obscure point connected with the early history of the human ovum concerns the first formation of the allantois, and the nature of the villi covering the surface of the ovum. The villi, if really formed of mesoblast covered by epiblast, have the true structure of chorionic villi; and can hardly be compared to the early villi of the dog which are derived from the subzonal membrane, and still less to those of the rabbit formed from the zona radiata.

Unless all the early ova so far described are pathological, it seems to follow that the mesoblast of the chorion is formed before the embryo is definitely established, and even if the pathological character of these ova is admitted, it is nevertheless probable (leaving Krause's embryo out of account), as shewn by the early embryos of Allen Thomson and His, that it is formed before the closure of the medullary groove. In order to meet this difficulty His supposes that the embryo never separates from the blastodermic vesicle, but that the allantoic stalk of the youngest embryo (fig. 168) represents the persistent attachment between the two[98]. His' view has a good deal to be said for it. I would venture, however, to suggest that Reichert's embryonic area is probably not in the two-layered stage, but that a mesoblast has already become established, and that it has grown round the inner face of the blastodermic vesicle from the (apparent) posterior end of the primitive streak. This growth I regard as _a precocious formation of the mesoblast of the allantois_--an exaggeration of the early formation of the allantoic mesoblast which is characteristic of the Guinea-pig (_vide_ p. 264). This mesoblast, together with the epiblast, forms a true chorion, so that in fig. 168, and probably also in fig. 164 A and B, a true chorion has already become established. The stalk connecting the embryo with the chorion in His' earliest embryo (fig. 168) is therefore a true allantoic stalk into which the hypoblastic allantoic diverticulum grows in for some distance. How the yolk-sack (umbilical vesicle) is formed is not clear. Perhaps, as suggested by His, it arises from the conversion of a solid mass of primitive hypoblast directly into a yolk-sack. The amnion is probably formed as a fold over the head end of the embryo in the manner indicated in His' diagram (fig. 168 _Am_).

[98] For a fuller explanation of His' views I must refer the reader to his Memoir (No. 232), pp. 170, 171, and to the diagrams contained in it.

[FIG. 168. DIAGRAMMATIC LONGITUDINAL SECTION OF THE OVUM TO WHICH THE EMBRYO (FIG. 165 A) BELONGED. (After His.)

_Am._ amnion; _Nb._ umbilical vesicle.]

These speculations have so far left Krause's embryo out of account. How is this embryo to be treated? Krause maintains that all the other embryos shewing an allantoic stalk at an early age are pathological. This, though not impossible, appears to me, to say the least of it, improbable; especially when it is borne in mind that embryos, which have every appearance of being normal, of about the same age and younger than Krause's, have been frequently observed, and have always been found attached to the chorion by an allantoic stalk.

We are thus provisionally reduced to suppose either that the structure figured by Krause is not the allantois, or that it is a very abnormal allantois. It is perhaps just possible that it may be an abnormally developed hypoblastic vesicle of the allantois artificially detached from the mesoblastic layer,--the latter having given rise to the chorion at an earlier date.

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