74 BULLETIN OF THE 



ing superadded to the surface of the dotted membrane, which last re- 

 sembles in all respects that of other fishes." "This outer covering," he 

 adds, " appears to be of cellular origin ; and Dr. Ransom thinks it may 

 be due to the separation of the tunica granulosa along with the ovum." 

 Thomson was able " to perceive the circle or lumen of the tubes " in 

 the dotted membrane by using a high magnifying power, and also 

 thought he could distinguish a hexagonal marking of the intervals be- 

 tween the pores (which he figures) in the salmon ; he also pointed out 

 that the size of these pores was only about one third of that of the tubes 

 in the perch as described by M tiller. The author was less successful, 

 when, in explaining Fig. 67*, he said: "A granular or dotted appear- 

 ance of these [granulosa] cells seems to indicate their conversion into 

 the dotted membrane, which is probably formed in successive layers from 

 the exterior. . . . The ovum (p. [103]) receives its firm porous mem- 

 brane [zona] while within the ovarian capsule, but only in the latter 

 part of the time of its formation." The origin of the membrane he 

 is incliued to connect with the interior of the ovarian follicle ; " but 

 whether by exudation from it, or by amalgamation of the innermost layer 

 of epithelial cells of the follicle," he has not been able to determine. 

 The latter he believes the more probable, and that -the membrane is the 

 true vitelline membrane. I am entirely unable to comprehend how 

 Thomson could have reconciled the two statements in the last sentence, 

 for surely the vitelline membrane was not, even at that time, regarded 

 as the product of anything but the ovum itself. 



On ripe fish-eggs Eeichert ('56, p. 89) was able to distinguish two 

 membranes, both of which were formed within the follicle. The inner 

 was the punctate membrane, but owing to the fineness of the markings 

 it was impossible to determine whether they were the result of elevations 

 or depressions of the surface. Reichert was unable to adopt without 

 reserve the conclusion that the dotted appearance is due to radial canals, 

 even though such an explanation was suggested by his finding the mark- 

 ings on the inner as well as the outer surface of the membrane in the 

 case of Cyprinus carpio. He evidently reposes great confidence in 

 Midler's explanation of the striation as an optical illusion, which in his 

 opinion accounts for the appearances figured by Aubert. He is also 

 uncertain in regard to the existence of a vitelline membrane, so that a 

 positive conclusion as to the nature of the zona radiata was not reached. 

 The smallest eggs possess a transparent homogeneous membrane without 

 punctations, and too thin to be measured, which may be regarded as a 

 vitelline membrane. With an increase in the size of the egg this mem- 



