678 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



to, in which the unripe ova were of variable size, large and small, would probably have 

 been retained until the end of winter or beginning of the year. The pelagic ova of this 

 species are amongst the most abundant forms in and beyond the bay in March, April, 

 and May. 



We have already spoken of the capsule as a zona radiata — a protective membrane of 

 general occurrence in the ova of most diverse groups of Vertebrates. Thus in the Aves 

 a zona radiata is present, though it does not persist ; but at an early stage it dis- 

 appears, and the egg leaves the ovarian follicle enveloped by another membrane which 

 is distinguished as the vitelline membrane. This second membrane is exceedingly 

 attenuate, so that it is difficult to distinguish it from the outermost layer of yolk- cells 

 from which it is derived. The Reptilia possess also two membranes ; but, unlike the 

 birds, they are not both of vitelline origin, the outer, which is very thin, Eimer (No. 53, 

 p. 418) declares to be a product of the follicular epithelium, and therefore chorionic ; but 

 the inner is thicker, and vitelline in origin ; and Eimer regards this as the zona radiata 

 (his zona pellucida). The capsule in the Amphibia (Rana) is a remarkable structure, 

 and would appear to be really a chorio-vitelline membrane, for the inner cells of the 

 ovarian follicle form a layer very closely applied to the true vitelline membrane, 

 and as the latter becomes continuously thinner the two layers are really inseparable, 

 and form one layer, of which the outer stratum is chorionic, and the inner one is 

 vitelline. 



In the Elasmobranchs a double layer is present, the outer being first formed, and 

 regarded by Balfour as vitelline ; while the inner one, equally of vitelline origin, he 

 distinguished as the zona radiata. Both, however, atrophy as a rule before the egg 

 leaves the follicle. In Cyclostomes (Petromyzon) two layers are described, an outer 

 imperforate, and an inner perforated layer. The outer layer on contact with water 

 swells up and forms a gelatinous coating by which the eggs adhere to external objects. 

 In Myxine, according to J. T. Cunningham's researches, the thick capsule is a chorion, 

 being developed along with its solid projecting processes from the follicular cells. 

 Possibly a delicate vitelline membrane may be developed internal to the outer homo- 

 geneous capsule, but this Mr Cunningham was not able to decide (No. 46, p. 600). 

 Notwithstanding that a double egg-membrane is so common, as indeed Kolliker long ago 

 pointed out (No. 80, p. 84), yet in the Teleostei the recognition of a single layer of vitel- 

 line origin accords best with the character of the capsule in general, in the mature ovum.* 

 Certainly Lereboullet's designation "chorion" (No. 93, p. 459) is inappropriate ;t nor 

 does Kupffer's view, that the capsule in certain osseous fishes is double, like the 

 Elasmobranchs, seem better justified. Kupffer holds that, in the case of Clupea 



* Dr Martin Barry affirmed that one membrane only envelops the ovum in fishes, no layer being formed 

 external to the vitelline membrane (No. 21, p. 309). Solger also came to the same conclusion from an examination of 

 Leuciscus rutilvs (Arch. f. Mik. Anat., 1885). 



t Lerebodllet, however, later in the same treatise, refers to the chorion as of vitelline origin, and "produced by 

 the primitive vitelline membrane separated from the vitellus " (No. 93, p. 507). 



