MAMMALIA. 



225 



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 raVjbit 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 charactei- 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 embiyo never separates from the 

 blastodermic vesicle, but that the allan- 

 toic stalk of the youngest embryo (fig. 

 168) represents the persistent attachment 

 between the two^ 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 

 blastodermic vesicle from the (apparent) 

 posterior end of the primitive streak. 

 This growth I regard as a 2if6<^ocious 

 forracitAon of the mesoblast of the allantois 

 — an exaggeration of the early formation 

 of the allantoic mesoblast which is cha- 

 racteristic of the Guinea-pig (vide p. 220). 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 choi'ion 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). 



These speculations have so far left Krause's embryo out of account. 

 How is this embryo to be treated 1 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 may be an abnormally 



^ For a fuller explanation of His' views I must refer the reader to his Memojr 

 (No. 232), pp. 170, 171, and to the diagrams contained in it. 



Fig. 168. Diagrammatic longi- 



TUDINAIi section OF THE OVUM TO 

 WHICH THE EMBUYO (fIG. 165 A) BE- 

 LONGED. (After His. ) 



Am. amnion; Nh. umbilical vesicle. 



B. E. II. 



15 



