42 K- ^IITSUKUEI; 



sections to he tlie primitice streak of tliis emhri/o and tlie groove that appears 

 like tlie posterior prolongation of the mediiUarg canal to he the primitive 

 groove. The primitive streah and groove therefore extend from the ncuren- 

 teric canal in front to the yolh-plug behind. At the posterior end, the 

 groove is pushed apart, as it were, to allow the yolk-plug- to 

 peep out on the surface. The mass formed by proliferation from 

 the primitive streak is mostly taken into the thick knob, the " End- 

 7cidst.'^ 



In this embryo, the yolk-plug extends over nearly sixteen 

 sections. It seems to be longer in Clemmijs than in the other two 

 species. 



Series IV. 



(PI. IV.) 

 Trionyx japonicus. 



An emhrgo with 4 mesohlastic somites ; taken out 5^ jo days after deposi- 

 tion ; represented in Fig. 19 (PI. III.) and also in Gontrih. II. PI. HI. 

 Figs. 17 CÙ 17a. . (i^s«)- 



In describing the surface changes, it was mentioned that Trionyx 

 behaves somewhat differently from Clemmys and Chelonia in the back- 

 ward movement of the yolk-plug, and that after passing through the 

 stage like Fig. 18 it becomes like Fig. 19, and the yolk-plug rests 

 flat on the posterior end of the medullary chord. This series is com- 

 posed of the selected transverse sections from the embryo represented 

 in Fig. 19. 



a is from the hinder portion of the mid-dorsal region. 



The medullary canal is still open and its wall continuous 



with the epiblast. 

 b is seven sections behind a. The chorda dorsalis is continu- 



