376 LOCW [Vol. VIII. 



The growth of the cephalic plate becomes more complicated, 

 for a frontal flexure progresses simultaneously with the other 

 changes, and while the head folds are undergoing various 

 transformations they are at the same time being drawn down- 

 wards in a new plane by the progress of this frontal flexure. 

 The "frontal flexure" has already begun in Fig. 2, and can be 

 seen in all the succeeding figures. It occurs so much earlier 

 than the flexure which has heretofore been designated the 

 cranial flexure that I use a definitive term for it. The central 

 tongue-like process, having become entirely separated from the 

 anterior tip of the body, is carried downwards by the infolding 

 to form the infundibulum and it becomes sickle-shaped in side 

 view. The head-folds begin to rise around the depressed 

 region in the forward part of the plate, while still retaining 

 their ventral flexure in the middle and hinder parts, and this 

 serves to further complicate matters. 



After the medullary folds become elevated in the head region 

 there is formed upon their margins a number of epiblastic 

 thickenings. I have not as yet traced their relation to the 

 central nervous system. 



The observations recorded above were made mainly upon 

 embryos of Sqiialus (Acajithias) vulgaris. 



There is one other point to which I wish to direct attention. 

 I have noted an invagination in the eggs of Galeus which 

 appears at a much earlier stage than any invagination that has 

 been recorded for other elasmobranchs. The involution occurs 

 at a stage that might be designated mid-segmentation period 

 (following the boundaries assigned by Balfour). It appears at 

 the posterior edge of the blastodisc, where the embryo finally 

 makes its appearance. 



Seen from above this invagination is crescentic in form and 

 covers rather less than 90° of the edge of the blastodisc. 



It resembles very strikingly the early invagination in 

 the bird's egg as figured by Kollar and also by Duval, and 

 the invagination in the reptiles as figured by Kupffer and 

 Benecke. 



I have seen this invagination many times on living eggs of 

 Galeus, and my sections show that it is a true infolding. 



