ON THE PROCESS OF GASTRULITION IN CHELONIA. 



239 



right up to the angle where the epi blast is reflected downwards at 

 the dorsal lip of the blastopore, and that the primitive knob has been 

 capped on to it from below, although now irrevocably fused with it 

 by a protoplasmic network. The floor of the cavity shows two 

 distinct divisions. In the posterior part (the vertical part in the 

 section) there is a compact mass of cells which have evidently been 

 proliferated from the floor of the cavity. This is the posterior median 

 part of the commencing peristomal mesoblast. In the anterior half of 

 floor, the vacuolated network ]jcomes very near the cavity, being 

 separated from it only by a thin sheet of cells. 



In the next stage (Figs. 8 and ,3a), we notice one striking 

 change in the ventral surface view of tlie embryo. While the top of 

 the primitive knob (spoken of with its ventral surface as uppermost, see 

 Fig. 3 a) is comparatively smooth as in the former stnge, its base 

 has assumed a lioney-combed structure and this structure is spreading 

 itself over the ventral surface of the embryonic shield. 



Fig. 14 is a longitudinal section near tlie median line of this 

 embryo. Compared with Fig. 13, the primitive knob has a longer 

 antero-posterior extension and it will be seen that this increase is due 

 almost entirely to the growth of the .'interior hnlf. The forward edge 

 of this half is gradually encroaching on the ventral surface of the em- 

 bryonic sliield (cf. Fig. 3 a) and is thus giving the primitive knoh ever 

 greater extension. Wenckebach (No. 15), AVill (Nos. 18 & 19), and 

 Mehnert (No. 8) agree in thinking that the forwai'd growth of the 

 priinitive knob takes place by its front growing edge insinuating itself 

 between the epiblast and the lower layer of the shield, and quit« indepen- 

 dently of these two sheets of cells.* My sections do not allow me to 



* In his latest paper (No. 21), Will admits that where gastruLitiou ia completed ]}j the 

 formation of the Kopffortsatz, the " primary " and " secondary " endoderm cells cannot be clearly 

 distinguished and that the former may grow ))y addition of the cells of the latter formed in situ 

 (p. 48). 



