94 PETER OKKELBERG. 



possesses a distinct membrane which has been termed by Lubosch 1 

 and others the zona radiata but to which I shall apply the more 

 general term vitelline membrane. In the ovarian egg of the 

 larva this membrane consists of a single layer but after meta- 

 morphosis it becomes differentiated into two distinct layers which 

 increase in thickness as the egg approaches maturity. The 

 outer layer usually shows faint striations which have been 

 supposed to represent canals. The two layers are very firmly 

 united. Beneath the vitelline membrane there is a cortical 

 alveolar layer which is covered on the outside by a very thin 

 protoplasmic layer (vitelline membrane of Lubosch). This thin 

 protoplasmic layer adheres very closely to the vitelline membrane 

 and is continuous with the rest of the egg protoplasm through the 

 walls of the alveoli. The alveolar layer seems to be similar to 

 that in Nereis as described by F. R. Lillie. 2 The same condition 

 has also been described by Reighard 3 in the egg of the wall-eyed 

 pike. I have not been able to find any perivitelline space in the 

 unfertilized egg. 



Outside of the vitelline membrane there is a gelatinous sub- 

 stance which becomes adhesive upon the addition of water. 

 This substance is supposed by Bohm 4 and others to be derived 

 from the follicular epithelium of the ovarian egg. It is thickest 

 at the vegetative pole and thins off gradually towards the 

 animal pole. Here it is covered by a mucous mass which was 

 first described by A. Miiller 5 who called it the "Flocke." It is 

 in the "Flocke" that most of the spermatozoa are found after 

 the egg is fertilized. The vitelline membrane is slightly modified 

 at the animal pole to form a region permeable to the spermatozoa 

 but there appears to be no true micropyle present. 



After fertilization visible changes almost immediately take 

 place which result in the separation of the vitelline membrane from 

 the egg and the formation of a large perivitelline space (Fig. 2). 

 This separation is initiated a short distance from the middle of 

 the animal pole as an indentation of the cortical layer, and a 



1 Lubosch, W., Jen. Zeitsch/. f. Naturwiss., Vol. 38, 1904. 



2 Lillie, F. R., Jour. Morph., Vol. 22, 1911. 



3 Reighard, Jacob, Tenth Biennial Report of the State Board of Fish Com- 

 missioners (Michigan), 1893. 



4 Bohm, A. A., Arch. f. Mikr. Ana'., Vol. 32, 1888. 



5 Miiller, A., Herrn Karl von Baer zur Feier des 50. Jahrestages, u.s.w., 1864. 



