FATE OF BLASTOPORE, ETC., IX CHELONIA. H^ 



to suppose to be the course of development in Lacerta, looking at 

 Will's fio^ures, is <is follows : — After such a stag^e as his Figf. 16 is 

 reached (in wliich Will is fortunate in having obtained an embryo 

 with the yolk-plug with its posterior boundary sharply defined), the 

 blastopore lips press towards the median line, and at the same time, 

 the posterior ends of the medullary fjids reach the yolk-plug and 

 embrace it between them. His Fig. 2 1 is prolmbly at or near 

 this stage. After this, the primitive streak and groove are poste- 

 riorly elongated, and pari jmssu with it the yolk-plug which is 

 always at the posterior end of the groove moves backwards. Will's 

 Figs. 22-27« show different stages in the progress of this event. 



While I can not thus accept Will's views in regard to the 

 closure of the bhistopore lips, I find his figures very clear and instruc- 

 tive. They present a series of pictures which fall in well with the 

 course of events I have described as occurring in the three species I 

 have studied. 



Feh. U, 1896. 



