MUSEUM OF COMPARATIVE ZOOLOGY. 73 



the true significance of Muller's discoveries in the perch. He retained the 

 name chorion for the zona radiata, and from a study of the trout was 

 fully convinced that the punctate appearance is due to " delicate tubules 

 or canals which traverse the membrane perpendicularly, without open- 

 ing, however, at its inner surface." The latter part of this statement 

 has not been confirmed by subsequent observers. Leuckart, however, 

 gave an excellent description of the structure of the zona radiata in the 

 perch, for he not only recognized that it was composed of an outer thin- 

 ner, firmer membrane, and an inner thick layer of viscid sarcode-like 

 substance, but he also saw that the two layers were so intimately joined 

 to each other that the canals were continued through both. While I 

 prefer to regard these two layers as substantially a unit, basing my con- 

 clusions on a variability in the apparent independence of the outer layer 

 and on the continuity of the pore-canals through both, I recognize that 

 this is a minor point, and that already Leuckart was in possession of the 

 important facts of structure. It was the presence of this " chorion " in 

 addition to Muller's capsule with its coarser pore-canals which convinced 

 Leuckart that the latter could not be considered the equivalent of the 

 radially striate membrane in other fishes, such as the salmon and trout. 

 I believe that Leuckart was less fortunate when he concluded that there 

 was in Esox a layer immediately outside the " chorion " which was homol- 

 ogous with the Mullerian " capsule " of the perch ; for in my opinion 

 there can be no doubt that the layer in question is the same as that in 

 which Eigenmann has found the pore-canals to be continuous with those 

 of the deeper portion of the zona. Even Leuckart describes the canals 

 as straight, not spiral as in Perca. In my judgment, therefore, this layer 

 corresponds to the thin outer layer of the zona seen by Leuckart in the 

 perch, rather than to the capsular layer described by Miiller. 



Although Volume V. (Supplementary Volume) of Todd's Cyclopaedia of 

 Anatomy and Physiology was not issued until 1859, the article " Ovum" 

 by Allen Thomson was published much earlier, and in two parts, ac- 

 cording to Gegenbaur ('61, p. 495). The first part (pp. 1-80) appeared 

 in 1852, and the second part (pp. [81]-[142]), which contains the por- 

 tion devoted to osseous fishes, in 1855. 



Thomson ('59, pp. [99], [100], [103]), besides giving a very brief 

 summary of previous work on the subject, treated at some length the 

 structure of the zona radiata, basing his conclusions partly on the work 

 of Ransom and partly on his own studies. On the strength of Ransom's 

 work he claimed that " the structure [Eikapsel] described by Miiller in 

 the perch was peculiar to that fish, and belonged only to an outer cover- 



