60 BULLETIN OF THE 



the margin of this area. Neither is it always the spermatozoon that 

 first reaches the outer membrane, after having traversed the " Flocke," 

 which passes through. 



The statement that the egg membrane is not alone permeable at a 

 single spot would lead one to suppose that the authors were ready to 

 deny the existence of a micropyle. They do not, however, directly assert 

 its absence, although they were unable to find anything of it on the un- 

 fertilized egg. But as soon as the spermatozoon has passed through the 

 membrane, a small circular spot may be seen from the surface ; this is 

 due to a shallow depression in the surface of the inner layer of the mem- 

 brane, the outer layer never showing any passage through it. The authors 

 hint at the possibility of a chemical action on the part of the spermato- 

 zoon resulting in a loosening of the two layers and a partial solution of 

 them, and endeavor to make that view harmonize with the conclusion 

 that the micropyle " is the remnant of an opening in the inner layer of 

 the egg membrane, which exists during the stay of the egg in the fol- 

 licle, corresponding to the condition which Herr von Jhering recently 

 established in the case of the eggs of the mussels." The outer layer 

 would be formed, they imagine, afterwards, and would cover over this 

 opening, leaving a remnant of it recognizable on the inner membrane. 



" The micropyle, therefore, is not an open passage, as it appeared from 

 Calberla's description and drawings, but only a permeable place." 



b. Selachii. 



What Ludwig wrote in 1874 concerning oogenesis in the selachians, 

 that it had been studied by only a very few investigators, was equally 

 true of the primary egg membranes of the group. Ludwig ('74, p. 145) 

 himself, although he studied the development of the ova, had nothing to 

 add to what was already known about the egg membranes, and since him 

 there have been only two writers who have dealt with the subject, 

 Schultz and Balfour. 



Leydig ('52, pp. 87, 88) speaks incidentally of a vitelline membrane, and 

 a thin albuminous layer surrounding it, in the case of Rajabatis. The latter 

 probably corresponds to one of the membranes seen by later observers. 



Gegenbaur ('61, p. 518) recognized the existence of a homogeneous 

 egg membrane on eggs of Raja from 1"' to 2'" (2-4 mm.) in diameter ; its 

 external contour was delicate, but internally it was sharply limited. In 

 the case of Acanthias there was only this one membrane to be observed; 

 it attained a thickness of 0.08'" (175 /x) on eggs 4" / -5'" (9-11 mm.) in 

 diameter. 



