FATE OF BLASTOPORE, ETC, IX CHELOXIA. 



93 



Woodcut XV. 



formation of the tail in Chelonia. The paired " Aniage " correspond 

 to the two sides of the primitive streak. 



(2) If we take a frog's eg<^, such as is represented in Woodcut 

 XY, A, cut it open at the h ne XY, and spread the whole flat, we shall 



obtain a figure like B. This reminds us 

 of the stages of Chelonia given in Wood- 

 cut IV, C (p. 12) or Contrih. IW PI. 

 YIII, Fig. 9. It is true that in Chelo- 

 nia, there is below the epiblast an under 

 layer, which I have called the cœnogene- 

 tic hypoblast, but as this has arisen 

 cœnogenetically around the secondary 

 yolk-mass (vide siipra)^ its absence in B is 

 of no consequence. If we stretch tliis 

 comparison one step farther, it makes 

 the conclusion highly probable that the 

 primitive plate or Inoh 

 (AVoodcut IV. A), acknow- 

 ledged by Will, ]\Ieiixert, 

 and myself to be present in 

 the two lavered stage of 

 Reptilia, corresponds to the loirer, Janjer-celled lialf of tlie anipliihian egg 

 or in otlier irords to tlie priiiiarg yolli-mass. It might be olijected tliat, in 

 the frog, gastrulation commences at the boundary between tlie upper, 

 and lower halves, or at the anterior edge of the lower half ii' spread flat 

 as in Woodcut XV, B, while gastrulation is stated to commence in the 

 middle of the primitive knob in Eeptiha. If we look at Fig. [) of my 

 Contrih. I ]\ it will be found that the gastrulation cavity is in the 

 middle of the primitive knoli, but as I have stated above (p. 13), the 

 epiblast spreads backwards to the edge of blastopore, simultaneously 



A section of a Frog's egg. 

 After AssHETON. 



The above figure cut open at XY and spread flat. 



