94 AKTHUR ROBINSON, M.D., AND RICHARD ASSHBTON, M.A. 



that the point x, which at this period marks the posterior 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 blastopore, 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). Ou 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 

 for as we have been able to ascertain, by the gradual withdrawal of 

 the yolk-plug and the rearrangement and differentiation of its con- 

 stituent cells. 



It is certainly noteworthy that the slit-like archenteron does not 

 appear in the midst of the yolk-cells until the epiblast reaches the 

 region of the blastoporic margin ; and it is this fact, together with 

 a predisposition to discover, if possible, an invaginative process, which 

 seems to have led to the description of the formation of the archenteron 

 of the Anura by invagination of the epiblast. 



A careful examination of the walls of the extending archenteric 

 cavity reveals no evidence in support of this ideal invaginative process ; 

 on the contrary, it shows that even at the period of completion of the 

 blastoporic opening (see figs. 1 and 3) the greater part of the cavity is 

 surrounded by large nutriment-laden, marginally pigmented cells. 

 More especially is this the case in the anterior portion of the, as yet, 

 incomplete cavity (fig. 2), and at its posterior extremity (fig. 3), where 

 the slit-like space is only just formed. In the latter situation the only 

 cell that can be considered as distinctly epiblastic is that marked c. 

 The remaining cells are undoubtedly large yolk-cells ; and it is only 

 by the division of these cells, and the arrangement of some or all of 

 their descendants into a distinct layer, that the true hypoblastic lining 

 of the alimentary canal and its diverticula is formed. As the com- 

 pletion of the definite hypoblast is intimately associated with the 

 separation of the mesoblast and chorda, its further history cannot at 

 this period be entered upon, and we may therefore proceed to the 



