FATE OF BLASTOPOEE, ETC., IX CHELOXIA. I'J 



Woodcut. V. 7 belongs. It is figured in its dorsal and ventral aspects 

 in Figs. 1 and la, PI. I. The head-fold has already sunk considera- 

 bly so that in the ventral view it ><tands out prominently as a trans- 

 verse ridge, marking the anterior end of the digestive canal. The 

 anterior amniotic fold is distinctly seen as a liorse-slioe shaped fjjd 

 although it leaves the head still exposed for the most p:irt. The 

 medullary groove is still very shallow, except in the posterior cephalic 

 res^ion where the medullary folds are raised hiadier than in front and 

 back. In the posterior region of the embrj'onic shield there is in the 

 median line a thick knob which stands out prominently on the ventral 

 view. This is directly derived from the earlier primiti\e knob, 

 althouo-h its nature has undero-one a considerable modification bv 

 various processes a.ccom])anying gastrulation and by the proliferation 

 of the peristomal mesoblast (cf Woodcut lY). ObH([uely through this 

 knob a canal passes from the dorsal to the ventral surface (Woodcut 

 IN . F). This canal Avhich I have called the blasto[)oric passage is the 

 posterior part of the archenteron which has not had its floor absorbed. 

 The dorsal opening of this canal, i.e., the blastopore, is now horse-slioe 

 shaped. The m;iss embraced within this horse-shoe is the structure 

 which we have homologised with the yolk-]>lug in Amphibia. The 

 ventral opening (jf the canal is rounded. 



The medullary folds now arise. Tlu-ee Tnouijx embryos (Figs. 

 16, 17, 18 W. in.) are introduced to illustrate the relations of the 

 medullary folds to the yolk-plug in the earlier phases of develop- 

 ment. As the medullary folds (which, it will be remembered, arose 

 first in the posterior cephalic region) proceed l)ack wards and ap|)roach 

 the region of tlie [)rimitive knob, which may now inore pro[)er]y Ije 

 called the terminal knob (Endwulst), they separate and enclose a. 

 wide space — the sinus rlioiiihoidalis (Figs. 16 & 17). When they reach 

 the yolk-plug, their posterior ends abut against the yolk-plug and 



