MUSEUM OF COMPARATIVE ZOOLOGY. 81 



would well repay one who should undertake it, for it could not be without 

 influence upon theories on the method of the growth of membranes. 



In his more extended paper Ransom ('68) gives an account of the 

 egg membranes of a number of fishes, but more particularly of Gaste- 

 rosteus, Esox, and Perca. To what he had already stated about the 

 " yelk-sac " of Gasterosteus he adds (pp. 440, 444, 448) that it is diffi- 

 cult, if not impossible, to determine the precise period at which it is 

 formed. In eggs gj^" (31.5 fx) in diameter it is not found, but is prob- 

 ably indicated by the smooth, hard outline which the yolk shows on its 

 surface. " It is separable in eggs ^W m diameter, and may be seen in 

 the fluids on the slide as a homogeneous-looking collapsed sac." With 

 a power of 500 diameters the dots appear round, and with one of 3,000 

 they are but obscurely hexagonal ; they are the same distance apart on 

 the inner surface as on the outer. Besides these minute regular dots, 

 there are larger and darker ones of a stellate form, which the author 

 suggests may in some way be connected with the interstitial growth of 

 the membrane. They occur irregularly at intervals of about ^oVo^j an< ^ 

 act like bodies of low refractive power; "at the cut edge they may be 

 seen to pass radially about two thirds into the substance of the yelk-sac, 

 gradually coming to a point and ceasing." I am not aware that any 

 other observer has confirmed this appearance, which I imagine may be 

 due to the presence of protoplasmic prolongations of the yolk into some 

 of the pore-canals, just as in Lepidosteus the substance of the villi is 

 traceable in many cases for some distance into the zona, although from 

 the opposite direction. 



"There are no facts known to me," says Ransom, "to point out 

 whether the pabulum for the growth of this membrane is derived di- 

 rectly from the currents passing inwards, or from the material elaborated 

 in the egg and passing out of it, or from both sources indifferently." 



Waldeyer (70, pp. 80, 81, 83), however, did not experience any such 

 uncertainty concerning the source of the zona radiata, for while he con- 

 tinued to call it the "Dotterhaut " and could find no vitelline membrane 

 inside it, he was very explicit in stating that it was a cuticular formation 

 produced by the follicular epithelium, and that the pore-canals were occu- 

 pied by delicate protoplasmic filaments which were in direct connection 

 with the epithelial cells of the follicle on the one side, and with the finely 

 granular yolk substance on the other. In his general conclusions con- 

 cerning the eggs of vertebrates he says : " The complete homology of the 

 zona pellucida [mammals] with the vitelline membrane of other verte- 

 brates can ... no longer be denied. The vitelline membrane is cer- 

 vol. xix. — no. 1. 6 



