MUSEUM OF COMPARATIVE ZOOLOGY. 87 



the zona radiata arises, whether as a product of the yolk or of the granu- 

 losa cells. An examination of maturing eggs shows him that its periph- 

 eral layers are always the most distinct. It is as though new layers 

 were being deposited from within, and this leads to the conclusion that 

 the zona radiata represents a true vitelline membrane. 



Although Hoffmann seems to me to come very near the true solution 

 of the problem, this presentation of the matter appears altogether un- 

 satisfactory, because he insists that what he calls the zona is practically 

 a unit in structure, and fails to recognize a fundamental difference be- 

 tween the outer villous layer and the " true zona radiata," as he terms 

 the inner portion of his zona. From this last conclusion one would be 

 led to infer that the zona (including both layers) was the result of a con- 

 tinuous process, and that there could not be any radical structural differ- 

 ence between villous and porous layer. But to my mind a common origin 

 from the yolk by no means implies identity of structure, nor the contin- 

 uous operation of the same formative process. 



Ryder ('81 c , '82 c , '83) has contributed a good deal to our knowledge 

 of the occurrence and functions of those modified forms of villi first 

 described by Haeckel, but I think he cannot have given their structure 

 very close attention or he would not have said ('83, p. 195) that "they 

 are apparently composed of the same tough material as that which enters 

 into the formation of the egg-membrane itself." He ('81 e , pp. 137, 138) 

 regards it as probable that the egg membrane (zona radiata) is secreted 

 from the cellular walls of the follicle. 



Since writing the above summary of and comments on the observations 

 of authors there have appeared a number of papers, some of them of con- 

 siderable importance, which I was unable to utilize in forming my opin- 

 ions of the nature of the membranes in Lepidosteus and other fishes. 



Stockman's ('83) account of the egg capsule in Salmo describes the 

 appearances of the pore-canals under a Reichert ^ homogeneous im- 

 mersion lens. In sections the limits of the pore-canals appear toothed, 

 owing to the presence of minute folds which have for the most part a 

 direction tangential to the capsule. These extend to the two ends of the 

 canal, and consequently its mouth appears angular rather than circular. 

 The substance of the capsule is beset with minute spaces which commu- 

 nicate with the pore-canals between the tooth-like projections, and are 

 believed to have a function in the transmission of nutritive material to 

 the egg. 



Ryder ('84, pp. 3, 4, 11, Plate I. Fig. 5) states that the cod's egg is 



