the Development of Anableps Gronovii. 441 
and grows until it acquires the length of an inch and a 
quarter, which gives the size of the longest fcetus which 
our specimens furnished. Even the umbilical sac and the 
fissure which sueceeds it, continue to grow after the yelk 
has disappeared. As a general rule among oviparous fishes, 
the yelk supplies al the material required for the growth 
of the fetus; and the same holds good with regard to 
nearly all Batrachians,* to scaly Reptiles and Birds. So 
general has this rule been believed to be, that none but Mam- 
mals have been supposed to contribute any thing beyond 
the materials of the egg to the support of the young. But 
recent observations go to prove that some fishes, such as 
the Torpedo among the Plagiostomes, the Embiotoca 
among osseous fishes, are to be placed in the same category 
as Mammals, in relation to the fact of being nourished by 
the parent during gestation, although neither a placenta is 
formed nor does any direct vascular communication what- 
ever exist between the foetus and the maternal circulation. 
We cannot explain the growth of the foetal Anableps by 
any other hypothesis than that it is nourished by a fluid 
Secreted by the walls of the sac in which it is lodged in the 
earlier stages, or by the parietes of the general ovarian cavity 
in which the foetuses are received towards the end of ges- 
tation. The high degree of vascularity of the egg-sac is 
favorable to this supposition. As the body of thé feetus, at 
à very early period, becomes covered with scales, absorption 
could only take place through the intestinal canal or by the 
surface of the yelk-sac, which invests the viscera and 
increases in size for a long period after the yelk itself has 
Wholly disappeared. In the later stages of gestation, even 
the Yelk-sac is out of the question, since it in turn who 
disappears, while the fcetus occupies the general cavity of 
ovary. 
E The only exception among Batrachians, as yet noticed, is found in the 
Pipe of South America. See Observations on Pipa Americana, by Jeffries 
Wyman, M. D., in American Journal of Science, 2d Series, vol. xvii. p- 309. 
JOURNAL B. S. N. H. 57 
