196 



THE ANCESTRY OF VERTEBRATES 



occurs, the beginning of which is already visible in fig. 1 (*). 

 Opposite the depression of the ectoderm a similar one is 

 found in the entoderm at the bottom of the anal diverticulum. 

 In an egg as represented in fig. 40 c we see, at 

 the bottom of the shallow invagination of the ectoderm 

 mentioned above, a little pit, as yet not very deep, from 



Fig. 40. Four eggs of Rana esculenta during the closure 

 of the medullary folds. 

 (a) anal pit, bl blastopore. 



which a still more shallow groove, the anal groove, runs 

 forward to the blastopore-slit. The longitudinal section 

 of the egg is given in fig. 3 of the plate. It bears a close 

 relation to fig. 2; the anal membrane has, however, become 

 thinner. 



