ON THE PROCESS OF ÜASTRULATION IN CHELONIA. 247 



the ]H*ogres.s made durino- the interval of time hetween the two 

 successive acts of takiii": oat, making' of course due allowance for 

 fast or slowly developing ego-s. Fig. 1 and Figs. 4-<S with some 

 intermediate staofes, whicli I ha\'e not introduced liere, helono- to one 

 of the series obtained in tliis mariner, and tliese show, conclusively 

 so my mind at least, that the area of the network spreads itself gradual- 

 ly underneath the eml3ryonic shield from the spot where the 

 invagi nation-cavity first breaks through, towards the periphery of the 

 thield i.e. from the posterior primitive knob anteriorly over the 

 embryonir shield. It is not the network that is gradually encroaching 

 on tlie central compact area, as Mehnert assumes, Init just the reverse; 

 for the central compact area is formed out of the area of the network. 

 The series, Figs, ôa — Ha, shows also that the area of the network 

 increases with the age of the embryonic shield. I can not therefore 

 accept Mehnert's explanation of the appearance of the network on 

 the xentral face of the blastoderm. 



As I said just now, I do not ))i"opose to go into the mesoblast- 

 formation again in tliis paper. I woidd merely remark that in tlie 

 stao-e corresponding to Fig. 17, I already see the establishment of the 

 chorda-hypoblast and the stretch of the epithelium on eacli side of it 

 which becomes transforjned into the gastral mesoblast. (Compare 

 Fig. 11, Contrib. TIL). 



There are two other points on which I wish to make some 

 remarks. 



The first of these is in regard to the position of the primitive 

 knob relatively to the embryonic shield. In Fig. 1, the primitive 

 knob lies for the most ])art outside of the embryonic shield, only 

 about one-third of its antero-posterior extension being within the 

 shield. In Fios. 2 and 3 it is al)out one-half, and in Fiiz'. 4. entirely 

 within the shield. This is no doubt brought about by the gradual 



