WITH SPECIAL REFERENCE TO THE VASCULAR SYSTEM. 



65 



the embryo. It is quite conceivable that the earlier normal curve might grade 

 insensibly into the later abnormal one. Wilson's specimen was, we imagine, very 

 near the parting of the ways, where it would have either gradually straightened out 

 or have suffered, through some influence, a much sharper degree of flexion. 



No attempt will be made to institute a formal and detailed comparison of No. 

 1878 with other embryos, nor to determine its developmental status as regards 

 them. Loose comparisons between embryos lose much of their apparent meaning 

 on account of the variability in rate and manner of development. To such varia- 

 tion we have recently drawn attention (1918) and fortunate or extreme instances 

 of it may yet throw considerable light on early human ontogeny. 



The greatest length of embryo No. 1878, measured on a straight line from the 

 anterior (later ventral) surface of the head just above the stomodeum to the cloacal 

 membrane, is, as determined on the model, 1.38 mm. Of this, rather more than one- 

 half will later be taken up into the head, the heart being now frankly in the head 

 region, while the rest of the future body is represented by the remaining posterior 

 part of the embryo, of which a small portion is occupied by the primitive streak. 



The elevation of the posterior end of the body is not due solely to the sinking 

 in of the region in front of it, but to a considerable extent to the developmental 

 changes which will very shortly entirely remodel this part of the embryo. The 

 anterior slope appears as a part of the general dorsal concavity, the posterior runs 

 down to the extreme end of the body close to the body-stalk, while its lateral sur- 

 faces fall off quite rapidly to the attachment of the amnion. Anteriorly and less 

 deeply over its summit it is grooved by the neural folds, which extend about one- 

 third the way down the posterior slope, of which a little less than the caudal half is 

 occupied by the primitive streak. The last-mentioned region, in part at least, cor- 

 responds to the steep declivity so well marked in the Glaevecke embryo of v. Spee 

 and the 1.3 mm. stage of Eternod. This portion, due to its active growth, will 

 not only contribute in length to the embryonic body but will also bring about the 

 so-called folding-off of its caudal extremity. The results of this process are the 

 formation of the hind-gut, the allantois then appearing as an appendage of the gut 

 instead of the yolk-sac, and finally the shifting to the ventral surface of the originally 

 dorsally placed cloacal membrane. Around the posterior margin of the tail-bud 

 thus formed, the primitive streak may persist for some time. These changes, due 

 to an active outgrowth rather than to any passive folding, are just beginning in No. 

 391 and are well under way in Kromer's embryo. 



Concerning the body-stalk and amnion a few words here will suffice. The 

 former is very large and massive, in fact much more bulky than the entire embryonic 

 body, and is roughly spindle-shaped with its smaller end attached to the chorion 

 quite obliquely. Its shape and size are due to the enormous vessels which it con- 

 tains, especially about its middle third, where it bulges prominently into the amniotic 

 cavity. On account of its relations to the body-stalk, the amnion shows two deep, 

 lateral recesses on either side of the stalk. From the left recess there passes out 

 into the stalk a long, irregular, hollow outgrowth which we may identify as an 

 amniotic duct (fig. 3). Its length, including the funnel-shaped origin from the 





