FATE OF BLASTOPORE, ETC., IN CHELONIA. 23 



groove as far as the groove surrounding the yolk-plug. The latter 

 structure is unusually distinct in this specimen and stands out pro- 

 minently as a white area. It is almost geometrically hexagonal, not 

 being marked by a well-defined groove only in its posterior boundary. 

 Its position some distance behind the " Endwulst " and at the edo-e 

 of the embryonic area is well shown in the figure, although there are 

 differences in some particulars from what is seen in Fig. 5 or 5b. 



There now ensues an important change, which Fig. 7 is in- 

 troduced to illustrate, as well as another point. Here is seen one of 

 those embryos in which a most careful search fails to reveal any trace 

 of the yolk-plug. The primitive groove can also not ]je seen. There 

 seems to be in different deposits of eggs great diversity in this res- 

 pect. For instance, the embryos belonging to the series from which 

 Figs. 5, 6, and 8 are taken show unusually clear figures of the yolk- 

 plug, the primitive groove, etc., while in the series to which Fig. 7 

 belongs, the yolk-plug and the primitive groove are obscured. 

 Histological details appear however to be essentially alike. The 

 change which Fig. 7 is intended to illustrate lies in the upheaval of 

 the tail-fold of the embryo. This is effected in the following way : — 

 The " Enchindst," which can be recognised by the fact that it surrounds 

 the posterior end of the medullary canal, becomes well marked in its 

 posterior boundary by the gradual rising vjnvanls of its own mass to 

 produce the tail-knob. I may here anticipate the results of a study 

 of the sections by stating that the tail-knob is not produced by the 

 tucking under of the blastodermic surface, but by an actual rising or 

 nphaeial of it at this part. The reasons for this conclusion will be 

 given in the part dealing with the study of the sections. A similar 

 figure (Fig. 25) in the Trionyx series will also illustrate this point. 

 Referring again to Figs. 5 and 6, we may therefore conclude that 

 the position to which the yolk-plug has retreated in these figures is 



