138 W. F. R. WELDON. 



side, which had separated from the axial hypoblast — all the 

 layers being, however, still fused in front of the blastopore. 



The statement of Kupffer/ that the blastoporic invagina- 

 tion gives rise to a closed sac, the walls of which become the 

 allantois, is of course inconsistent with the truth of the above 

 observations; but it has been already so abundantly disproved, 

 first by Balfour and afterwards by Stahl and Hoffmann, that 

 it is not necessary here to do more than refer to it in passing. 



The actual mode of development of the allantois was first 

 figured by Balfour,^ a copy of whose drawing is repioduced 

 in the woodcut. The details of the process were worked out 

 by Strahl.^ 



I have nothing to add to the account given by these authors, 

 but I would call attention to a consequence of it which neither 

 observer has, to ray knowledge, remarked. . 



It is obvious from the woodcut that, as has been shown in 

 detail by Strahl,* the allantois arises as a process of the 

 primitive streak, which projects at first backwards into the 

 body cavity. 



Now, if this be the case, when the primitive streak is bent 

 ventralwards during the establishment of the tail fold, the 

 primitive streak must extend in the middle line from the 

 posterior extremity of the medullary canal, round the end of 

 the embryo, as far forwards as the point of connection of the 

 allantoid stalk, with head cut ; and therefore the proctodoeum, 

 when it arises, must not pass through the primitive streak. 



Therefore, if we adopt the view of Balfour, that the primi- 

 tive streak represents the position of the blastopore of other 

 gastrulae, we shall be forced to conclude that, at aiiy rate in 

 this group of Craniata, the anus is in the position of a part of 



> Kupffer, " Die Gastrulation an den Meroblasiisclien Eiern der Wirbel- 

 tliiere uud die Bedeutung des Primiliv Streit," ' Arch. f. Anat. u. Pliys.,' 

 1882. 



*■' " On the Early Development of the Lacertilia," &c., this Journal xix. 



' Strahl, " Ueber die Eutwicklung des Canalis Myeloenteriens und der 

 Allantois der Eidechse," ' Archiv. f. Anat. u. Phys.,' 1882. 



* Loc. cit. 



