ON THE PROCESS OF GASTRULATION IN CHELONIA. 



265 



Post 



> Ant. 



Edge of the Sickle. 

 Fig. B. 

 ■ Fig. B. Posterior part of a. longitudinal section of the Embryo given in Fig. A. 



with the îippearance of the sickle in this series of sections, I was able to 

 detect the same structure persisting in the sections of some older embryos 

 of CJemmys. There i.^ ;i great deal of variation in the degree of 

 development to which this structure attains in different individu;ds as 

 well as on the two sides of the same individual. It seems to disappear 

 entirely later. As the mesoblast develops afterwards quite independent- 

 ly of this, it is not what Will calls Kuplfer's sickle. For tlie present, 

 I think, it corresponds probably to the sickle (Koller's si(.'kle) which 

 Will describes in the earliest stao-e, althouu'h there are some features of 

 it which I do not yet quite ccmiprehend and which may fin;dly esta- 

 bUsh its ditference from Koller's sickle. 



Aj'arr from the structure which I have described, I can detect 

 nothinii' comparable to Koller's sickle in mv materials, 



2. Will makes out a sharp line of demarkation between the 

 ectoblast and entoblast at the edge of the primitive plate (1 Art. Figs. 

 43, 44, and others ; II Art. Figs, lo a. and />.). Since reading Will's 

 second article, I have again carefully gone over the sections of my 

 earliest stages, but J am unable to make (nit sucli a line at all. As 

 this line is figured in AVill's papers as pei-sisting to quite late stages, 

 I aui surprised that I do not see it at least in some of my sections, if 

 it really exists. 



,'). Perhaps the most serifjus jxnnt of ditference in the observations 

 of Will and of myself is in regard to the extension of the invagination 

 cavity, before it breaks through below. In Cistudo, Will states that 



