2/O HUMAN OVUM. 



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 blasto- 

 dermic vesicle, but that the allantoic 

 stalk of the youngest embryo (fig. 168) 

 represents the persistent attachment be- 

 tween the two 1 . His' view has a good 

 deal to be said for it. I would venture, 

 however, to suggest that Reichert's em- 

 bryonic 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 FlG l68 DlAGRAMMATIC LONGI . 



blastodermic vesicle from the (apparent) TUDINAL SECTION OF THE OVUM TO 

 posterior end of the primitive streak. WHICH THE EMBRYO (FIG. 165 A) BE- 

 LONGED. ( After His.) 

 This growth I regard as a precocious 



.. ,. ..., ,, , ,.., ,, , . Am. amnion ; No. umbilical vesicle. 



formation of the mesoblast of the allantozs 



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. 1 68) 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}. 



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 ap- 

 pearance 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 maybe 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. 



1 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. 



