HABITS, MORPHOLOGY, AND EMBRYOLOGY OF GAMBUSIA AFFINIS. 1 83 



condition presses forward against the stomach, while at the right it extends anteriorly 

 alongside the latter organ. (PI. xvi, fig. 8.) 



Unlike the ovary of many oviparous teleosts, the ovary of Gambusia is not lobulated 

 and contains relatively few ova. In the same ovary may be found ova in various stages 

 of development, ranging from almost microscopic dimensions to a diameter of 1.8 milli- 

 meters attained at maturity. A considerable number of ova apparently reach maturity 

 at the same time. These being fertilized give rise to a brood of young. After the birth 

 of this brood, another lot of ova reach maturity, and, being fertilized, give rise to a second 

 brood. Thus, perhaps, all the ova required to produce the several broods which are 

 born during a spring and summer may be present in the ovary at the beginning of the 

 season. 



The larger females usually give rise to a larger brood of young than do the smaller 

 ones. The average number of embryos contained in the ovaries of females 5 to 6 centi- 

 meters in length, based on a limited number of counts, was found to be 33. The maxi- 

 mum number removed by the writer from a single female was 76. The number of 

 embryos contained in the ovaries of the smaller females ranges from 2 or 3 to about 20. 



In females of this species taken in the Potomac River early in June, 191 2, Smith " 

 found the average number of embryos contained in the ovary to be 100. This average 

 is considerably greater than the maximum number observed by the present writer. 

 This difference is probably due to the fact that the broods observed by Smith were the 

 first broods produced during the season, while those observed by the present writer 

 were the second or later broods. As suggested by Smith, the first brood of the season 

 is probably considerably larger than the later broods. 



The ova of Gambusia have no investment of their own save a delicate vitelline 

 membrane. Each ovum is inclosed in a separate cellular follicle which is attached to a 

 central rachis (Ryder) by a slender stalk. Running longitudinally in the central rachis 

 are a pair of vascular trunks from which smaller blood vessels arise and pass out along 

 the stalks of the follicles. These smaller blood vessels break up into capillaries which 

 radiate in all directions over the follicular walls. These follicles were described by 

 Ryder (1885) as follows: "The ovarian follicles of Gambusia containing mature ova or 

 foetuses are built up internally of flat or squamous polygonal cells of pavement epithe- 

 lium, and externally of a network of multipolar fibrous connective tissue cells and 

 minute capillary blood vessels with cellular walls, which radiate in all directions over 

 the follicle. From the point at which the main arterial vessel enters it, this vessel, 

 together with its accompanying vein and investment of fibrous tissue, constitutes the 

 stalk by which the follicle and its contained naked ovum is suspended to the main 

 arterial trunk and vein." * 



In an earlier paper, Ryder "^ (1882) has furthermore described a minute aperture 

 in the follicular membrane near the stalk of the follicle which he has designated "the 

 follicle pore." Through this pore, he believes, the spermatozoa enters the follicle. 



I was able satisfactorily to observe such a pore in the follicular membrane in only a 

 few instances. I have no reason, however, to doubt the presence of a follicular pore 



o Smith, H. M.: The prolificness of Gambusia, Science, vol. xxxvi, n. s., no. 920, 1912, p. 224. 



b Ryder. John A.: On the development of viviparous osseous fishes. Proceedings of theU. S. National Museum, vol. vin, 

 p. 147. 



c Ryder, John A.: A contribution to the embryography of osseous fishes, with special reference to the development of the 

 cod {Gadus morrhua). Report of the U. S. Fish Commission, 1882, p. 461. 



