466 ARTHUR ROBINSON AND RICHARD ASSHETON. 



iu the first phases of its development by large yolk-cells, 

 which eventually give rise to the definite hypoblast. 



5. The ventral lip of the blastopore indicates the posterior 

 end of the primitive ventral wall of the archenteron. 



6. There is no ventral and forward extension of the archen- 

 teron in front of and below the ventral lip of the blastopore 

 by the production of a diverticulum in that situation. 



7. The ventral wall of the archenteron, in front of the ven- 

 tral lip of the blastopore, is completed by the extension of the 

 anterior end of the cavity, and by the withdrawal and modi- 

 fication of the cells of the yolk-plug. 



Manner in which the Anus of Rusconi closes. 



Having discussed the changes that take place in the ovum 

 up to the time of the formation of the large circular blastopore 

 or anus of Rusconi, we will now proceed to describe the 

 manner in which, according to our observations, the closure of 

 the blastopore or anus of Rusconi seems to be effected, and to 

 give in some detail an account of the structures which are 

 the immediate result of the method of closure about to be 

 described. 



According to some of the former accounts, to which we have 

 made reference above, the anus of Rusconi has been said to 

 diminish in size by the gradual coming together of each portion 

 of the blastoporic rim simultaneously. This we believe to be 

 incorrect. No doubt from surface views alone the hitherto 

 accepted accounts receive much support. 



We have, however, paid particular attention to this point, 

 and have come to the conclusion that the anus of Rusconi 

 gradually diminishes in size by the concrescence of the ventral 

 part of the lateral lips, as indicated in figs. 10 and 18. In 

 these diagrams, the space lettered BR, inclosed between two 

 concentric rings, represents the blastoporic rim at the time of 

 the completion of the circular blastopore, or anus of Rusconi ; 

 and the shaded space, lettered BR, represents in fig. 10 

 the position of the blastoporic rim soon after the anus of 

 Rusconi has begun to diminish in size, and in fig. 18 a later 



