MUSEUM OF COMPARATIVE ZOOLOGY. 149 



The eggs examined by me may be divided as follows : — 



I. Eggs with a single membrane, the zona radiata. 



a. Zona radiata a single layer of uniform structure. Notemigonus 

 chrysoleucus, Carassius auratus. 



aa. Zona radiata differentiated into an inner and outer layer. Morone 

 americana, Esox reticulatus, Cyclogaster lineatus, Amiurus catus. 



II. Eggs with a zona radiata and a thin homogeneous outer layer. 



b. Outer membrane without appendages. Clupea vernalis. 



bb. Outer membrane bearing filiform appendages. Fundulus hetero- 



clitus, F. diaphanus. 

 bbb. Outer membrane with short appendages. Pygosteus pungitius. 



III. Eggs with a zona and a thick outer layer produced by a secretion 

 from and metamorphosis of the granulosa cells. Perca americana. 



Origin of the Egg Membranes. 



Concerning the origin of the different egg membranes of fishes several 

 views have been held. 



Vogt ('42) and Vogt and Pappenheim ('59) maintained that the zona 

 radiata is formed by the compression of a layer of cells surrounding the 

 egg; Reichert ('56), Kolliker ('58), Gegenbaur ('61), and Eimer (72 a ), 

 that it is derived from the yolk ; Thomson ('59) and Waldeyer ('70), that 

 it is derived from the follicular epithelium ; Ransom ('67) argued that it 

 cannot grow by apposition of layers from within or without, and that it 

 must grow by interstitial deposition of material. Whether this material 

 comes from ingoing or outgoing currents, he was unable to determine. 



I think that the zona is undoubtedly derived from the yolk. Kolliker 

 found that in all the filament-bearing eggs studied by him the zona 

 radiata was formed after the filament-bearing membrane. I have found 

 the same to be true in Fundulus. In the case of Morone the outer layer 

 of the zona does not become much thicker after the inner layer has begun 

 to be formed, whereas the latter continues to grow rapidly. In the case 

 of Cyclogaster lineatus, where the outer layer of the zona shows columnar 

 structures, these do not bear any definite numerical relation to the over- 

 lying cells of the granulosa. The outer portion of the zona is almost 

 always more uniform in its structure, and stains deeper, than the inner 

 portion. 



Reichert ('56) and Kolliker ('58) are inclined to believe that the cap- 

 sular layer of the perch is derived from the granulosa cells, an opinion 



