68 BULLETIN OF THE 



observed which can hardly bear any other interpretation than that of a 

 striate zone of substance inside the zona radiata proper ; but in other 

 cases (Esox, Amiurus) a somewhat similar though perhaps not identical 

 appearance is due to a retraction of the vitellus from the zona, which 

 leaves strands of vitelline substance stretching across the space thus 

 produced. 



1. Zona Radiata and Villous Layer. 



Johannes Muller ('54) has often been credited with having discov- 

 ered the fine radial canals which traverse the zona radiata, and give to it 

 its most characteristic appearance. This is a mistake which has arisen 

 from Midler's misunderstanding the relation of the peculiar membranes 

 of the perch to those of other fishes. What he described as " pore- 

 canals " in Perca belonged to a much thicker membrane than the zona. 

 This membrane lies outside the latter, and is a result of the activity 

 aud metamorphosis of the granulosa cells. Midler, it is true, supposed 

 this to be the equivalent of the " shell membrane " previously described 

 by Vogt, and therefore imagined that he had been able to demonstrate 

 on a more favorable object what Vogt had claimed on grounds of analogy 

 rather than on satisfactory proof. 



In Vogt's studies on Coregonus palcea he ('42, pp. 1, 8-10, 27, 28) 

 claimed the presence of two membranes. The inner one — being thin, 

 transparent, and without apparent texture — he called vitelline mem- 

 brane ; the outer one he called a shell membrane, and homologized it 

 with the "membrane coquilliere " of birds' eggs. This outer membrane 

 presented the appearance of shagreen, which seemed to result from a 

 quantity of small opaque points uniformly distributed over its surface. 

 Treated with hydrochloric acid, the points became more transparent, 

 and then resembled minute warts. Valentine called Vogt's attention to 

 the resemblance between this structure and that of the carapace of the 

 cray-fish, where he had found that a similar effect was due to perpen- 

 dicular tubes, filled with lime, traversing a membrane composed of regu- 

 larly polyhedral cells. Vogt, admitting that the "shell membrane " was 

 too thin to allow the attainment of exact results relative to the nature 

 of the "points," nevertheless claimed that the position, behavior, and 

 reticulate appearance of the latter warranted one in supposing that the 

 structure was analogous to that of the carapace of the cray-fish. Thus 

 it appears, he continues, that the shell membrane is formed by the union 

 of flattened cells, which are arranged around the primitive egg only 

 toward the epoch of its maturity ; the presence of these minute tubes, 



