I38r..] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 145 



^ 



Vol. VIII, Wo. 10. IVashifi^ton, D. C. -^lay 35, 1885. 



also afterwards enters into the structure of the walls of the ovarian 

 sacs or follicles externally, as these grow in size. 



The immature ova measure from a hundredth to a fiftieth of an inch, 

 and enlarge to something more than a line in diameter, when they may 

 be said to be mature. The median vessel of the ovary extends back- 

 ward and slightly "downward, and arises very far forward in the roof of 

 the body-cavity from the dorsal aorta. 



The ova after developing a little way are each inclosed in a follicle 

 or ovisac, memhrana granulosa of Von Baer, or memhrana cellulosa of 

 Coste. As the egg develops there seems to be a space gradually formed 

 about it in the same way as described by Wyman in Anahleps. This' 

 space is filled with fluid, and in this liquid, which increases in quantity 

 somewhat as development proceeds, the embryo Cyprinodont is con- 

 stantly bathed. 



There is no trace whatever in the egg follicles of Gambusia of an inde- 

 pendent egg membrane^ such as is present in the ova of all known forms 

 of osseous fishes which spawn directly into the water, the egg mem- 

 brane or zona radiata of the latter, in all cases being perforated by a 

 minute pore or micropyle for the passage of the spermatozoa. The 

 condition in Gambusia raises the question whether the zona is not more 

 or less completely aborted, sooner or later, in all vivijjarous fishes. 

 Wyman, however, points out that a membrane exists, covering the 

 immature ovarian eggs of Anableps, and I find that the mature eggs of 

 Fu7idulus are provided with a strong zona radiata during the early 

 stages of» their intra-ovarian development ; how long it persists, how- 

 ever, has not been determined, as it is possible that it is disintegrated 

 and absorbed during the later stages of gestation. Eathke's descrii^tion 

 of the ovarian follicles of Zoarces agree in some respects pretty closely'" 

 with my account of the egg-follicles of Gambusia given farther on. 

 The narrow elongate stigma, devoid of vessels, on the follicle, spoken 

 of on page 4 of his memoir on Zoarces, probably corresponds to what 

 I have called the follicular pore. He has described a vascular net- work 

 in the follicle, a stalk joining it to the vascular rachis, and a space 

 around the yelk much as in Gambusia. 



The zona radiata, or covering of the eggs of osseous fishes, is said to 

 be secreted from the cells lining the follicles, and is composed of a strong, 

 somewhat gelatinoid substance, and is usually, though not always, per- 

 forated all over by a vast number of extremely fine tubules, called joore 

 canals by tbeir discoverer, Johannes Mueller. In the yellow perch 

 (Percaflavescens), the zona radiata is well marked, but external to it 

 there is a highly elastic layer many times thicker, which is traversed 

 by what seem to be tubules, which are, however, much less numerous 

 than those of the true zona. In the ova of closely allied forms, such as 

 the white perch and r ck-fish or striped bass, this extra external envel- 

 Proc. Nat. Mus. 85 10 



