132 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. [188J. 



these specimeus it measured 1 iucb iu length, aud was consequently 

 longer than the whole embryo of the first specimen noticed above. The 

 edges of the fissure were united by the intermedium of a thin membrane, 

 without scales, on which no papillas were noticed, and was sufficiently 

 lax to allow the edges of the fissure to separate from each other to a 

 slight extent. The transverse baud upon the cornea was now distinct, 

 though it had not yet become as opaque as in the adult. 



" Seven foetuses were found in the ovary ; on the sides of them one or 

 two dark longitudinal 1 nes were now visible; the general form of the 

 body had assumed more precisely that of the adult, and, as noticed by 

 Valenciennes, the intestines had obtained their permanent form. The 

 external sexual characters were not visible in any of the specimens ex- 

 amined, though they were seen and figured iu specimeus of about the 

 same size by Valenciennes. 



''All of the foetuses of this female had escaped from their original 

 sacs (no traces of which were now visible) and were all contained in one 

 large cavity formed by the dilated ovary which now had become analo 

 gous to an uterus and extended from the genital opening as far forwards 

 as the bases of the pectoral fins. The walls of this ovarian sac were suf- 

 ficiently thin to allow the foetuses to be seen through them. On its in- 

 ner surface, as well as ou that of some of the other specimens, were to 

 be seen numerous immature ova, some of them microscopic and others 

 as large as the sixteenth of an inch in diameter. The co-existence of 

 immature ova on the walls of the ovarian canity with foetuses in it cor- 

 responds with wjiat was noticed by Duvernoy in his investigations of 

 the embrj'ology of Poecilia.* The more minute ovarian eggs, though 

 for a long time macerated in alcohol, yet preserved their microscopic 

 characters to a remarkable degree. The smallest consisted of a cell, in 

 the center of which a nucleus was visible, and around this last were a 

 few granules. In the larger ova the granules have become more and 

 more abundant, and in some instances obscure the nucleus or germ-cell. 

 After the egg has increased to a certain size, a clear space appears ex- 

 terior to the vitelline membrane, which gradually increases to nearly 

 twice the diameter of the egg itself. This clear space is limited by the 

 substance of the stroma, which becomes condensed around it and thus 

 forms a distinct sac. If the ovum be compared to that of a mammal, 

 then the sac just described may be said to be analogous to a Graafian 

 vesicle ; that is, the egg of the fish floats free in a sac much larger than 

 itself, just as the mammiferous egg does in the vesicle of De Graaf. 

 There were no intermediate conditions between this and the impreg- 

 nated condition to enable me to determine whether or not it is this sac 

 which foimed the external covering of the foetus. Valenciennes seems 

 to adopt the idea that it does, and compares it to a chorion.! If this view 



* Sur la tleveloppemeut de la Poecilia snrinamensis, Ann. des Sci, Naturelles, 3« ser., 

 i, p. 313, 1814. 



tLa cellule qui contieot un oeuf f6cond6 s'aggrandit et finit par former uno 8orte 

 de chorion. Ojj. cit., t. xviii, p. 261. 



