1885.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 133 



of its nature be true, then there seems no alternative, since develop- 

 ment advances so far before the sac ruptures, but to suppose that im- 

 pregnation must take phice through its parietes and that the sperma- 

 tozoon cannot enter bodily into the substance or even come in direct 

 contact with the vitelline membrane of the egg except through the walls 

 of its outer covering, which is not probable. It would seem that it must 

 act simply by its presence on the surface of the egg-sac or by anendos- 

 mosis of its Huid contents through the membranes by which the ovum 

 is invested.* 



"A microscopic examination of the egg-sacs in the advanced foetuses 

 proves conclusively that they do not consist of loose areolar tissue only, 

 as stated by Valenciennes,! but that while the tissue in question forms 

 the basis of them, they are in reality highly vascular, large trunks and 

 minute ramifications of vessels being easily traced by the aid of the 

 coagulated blood which they contain. 



" In comparing fcetuses of different stages of development together, 

 a very interesting question is i^resented to us in connection with their 

 growth. In the smallest si)ecimen examined, the yelk was no longer 

 visible; it had been wholly consumed in supplying materials for the for- 

 mation of the embryo; and yet subsequent to this disappearance of the 

 yelk, the embryo, while still in its ovarian sac and cut oft" from all ex- 

 ternal communicatioQ, continues to increase in size, and grows until it 

 acquires the length of an inch and a quarter, which gives the size of 

 the longest foetus which our specimens furnished. Even the umbilical 

 sac and the fissure which succeeds it continue to grow after the yelk 

 has disappeared. As a general rule among oviparous fishes, the yelk 

 supplies all the material required for the growth of the foetus; and the 

 same holds good with regard to nearly all Batrachians,]: to seal}' rep- 

 tiles and birds. So general has this rule been believed to be, that none 

 but mammals have been supposed to contribute anything beyond the 

 materials of the egg to the support of the young. But recent observa- 

 tions go to prove that some fishes, such as the Torpedo among the 

 Plagiostomes, the Embiotoca among osseous fishes, are to be jilaced iu 

 the same category as mammals, in relation to the fact of being nourished 

 b}' the parent during gestation, although neither a i)lacenta is formed nor 

 does any direct vascular communication whatever 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 gestation. The high de- 

 gree of vascularity of the egg-sac is favorable to this supposition. As 



[*See description below of the follicular pore in Gambusia.!; 



tO^x cj<., p. 261. 



t The only exception among Batrachiaus, as yet noticed, is found in the Pipie of 

 South A.uierica. See Observations on Pipa Americana, by Jeffries Wyman, M. D., in 

 American Journal of Scieucej 2d series, vol. xvii, p. 369. 



