154 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. [1885. 



Conclusion. — The foregoing account greatly expands and finally com- 

 pletes the history of the development, as far as my materials permit, of 

 this remarkable form, preliminary notices of which I i)ublished in For- 

 est and Stream in August, 1881, and in the American Naturalist* for 

 February, 1882. While this history is still far from complete, especially 

 as respects the very early stages, we have at least cleared the ground 

 and prepared the way for further investigations. A number of points 

 which had been left undecided have been determined. These are the 

 following : 



1. That fertilization of the egg of Gamhusia occurs within tlie ovarian 

 follicle, the spermatic fluid being apparently introduced into the ovary 

 or abdominal cavity by the male, which is provided with an intromittent 

 organ consisting of the anal fin much modified, and that the spermatozoa 

 find access to the egg through a wide opening in the follicle which 

 answers to a micropyle, but which may be called the foUicidar pore. 



2. That tliere is no evidence, as in the case of Anableps and the 

 Umbiotocida', that the ovarian follicles are ruptured until the develop- 

 ment of the young embryos is approximately completed, since the most 

 advanced foetuses of Gambiisia studied by me have the scales, fins, fin- 

 rays, and cranium remarkably well developed, even before the yelk is 

 all absorbed, as it is known that the young are born not a great while 

 after the stage of development represented in Fig. 3 has been reached. 



3. It is also known that little or no nutriment is derived from the 

 parent, as in Anableps and the Embiotocidw ; or, In other words, that the 

 embryo of Gambnsia grows entirely at the expense of the material con- 

 tained in the j; elk-sac, and does not form villi upon the latter nor en- 

 large after the yelk has been absorbed as in Anableps; neither does the 

 rectum or hind gut hypertrophy, nor do the fins expand and develop 

 prolongations of the interradial membranes as in the Embiotocida\ 



4. As is the case with all viviparous forms, the number of embryos 

 produced seems to be diminished in correlation with the protection 

 Avhich the young receive in consequence of their peculiarly complete 

 development within the body of the parent. The embryos leave the 

 parent as active little fishes about half an inch in length ; in fact this 

 fish begins an independent career as far developed as when the shad, 

 cod, Spanish mackerel, cattish, and many other fishes are from three to 

 six weeks old. By so much it has the advantage over those species in 

 the struggle for existence in that it is ready to feed, to pursue its prey 

 discriminately, as soon as it is born, while the other forms alluded to 

 are comparatively helpless until some time after they are born ; hence 

 nature makes up in the fertility of such species for the advantages 

 enjoj'ed by the viviparous ones, so that finally the chances of the 

 survival of the two types, which differ so widely in their breeding 

 habits, are about equal. 



* Structure and Ovarian Incubation of Gambusia patruelis, a top-minnow. Am. 

 ^at., 1882, pp. 109-118, 



