464 ABTHUE EOBINSON AND RIOHAED ASSHETON. 



at the same time defines the posterior limit of the ventral wall 

 of the archenteron (fig. 3). 



A diagrammatic representation of a sagittal mesial section of 

 the ovum at the period of completion of the blastoporic lip is 

 given in fig. 4, from which it will be readily seen that, at this 

 period, the ventral wall of the archenteron extends from the 

 point a to the point x. In a portion of its extent the floor of 

 the slit-like cavity lies parallel to the superficial surface of the 

 ovum, but posteriorly it is projected outwards into a deficiency 

 in the external wall (the blastopore), forming in this region the 

 yolk-plug or bouchon d'Ecker. At the same stage the dorsal 

 wall of the archenteron is not co-extensive with the ventral. It 

 extends only from the point a to the point 6, which marks the 

 dorsal lip of the blastopore. From the dorsal to the ventral 

 lip of the blastopore the wall of the archenteron is deficient. 

 It will be shown afterwards how this portion of the wall, which 

 we shall call the posterior wall of the archenteron, is completed. 



At present we more particularly desire to call attention to 

 the fact that the point x, which at this period marks the pos- 

 terior limit of the ventral wall of the archenteron, remains 

 fixed throughout the later periods, that eventually it is situated 

 just in front of the anal orifice, and that there is no extension 

 ventrally of the archenteron beyond it, such as that described 

 by Balfour, who, speaking of the gradually narrowing blasto- 

 pore, says : " At its front border, on the ventral side, there 

 may be seen a slight ventrally directed diverticulum of the 

 alimentary tract, which first becomes visible at a somewhat 

 earlier stage '^ (1, p. 108). On the contrary, the ventral wall 

 of the archenteron is only increased in front of the ventral lip 

 of the blastopore by the forward extension of the anterior end 

 of the cavity and the growth of the ovum, and is completed 

 immediately in front of the ventral lip, so far as we have been 

 able to ascertain, by the gradual withdrawal of the yolk-plug 

 and the rearrangement and diflferentiation of its constituent 

 cells. 



It is certainly noteworthy that the slit-like archenteron does 

 not appear in the midst of the yolk-cells until the epiblast 



