328 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



poured into the same vessel with the latter, impregnation taking place 

 at once.* 



From that moment the successive phenomena of development are not 

 easy to follow under the microscope. We find, after the generative 

 elements are brought into contact, that the egg, which was at first 

 pear-shaped, becomes more and more nearly round; the germinative 

 vesicle is effaced and disappears totally, the polar globules appear at 

 one point on the periphery, the egg segments into two, three, and four 

 parts, the number of which goes on increasing until it finally assumes 

 the mulberry or morula form. In order to clearly describe the trans- 

 formations of the egg, it would be necessary to have illustrations ap- 

 pended. In the course of seven or eight hours of incubation, accord- 

 ing to the temperature, the embryos begin to move, and a mobile larva 

 appears.t 



The mobility of the larva is manifested in accelerated movements of 

 rotation or by sudden starts across the field of view in which they are 

 observed. Sometimes they rotate as if on a pivot, sometimes they re- 

 main quite immobile; but if we examine them with care we find that 

 the vibratile cilia with which they are provided continually manifest 

 their peculiar motions. Arrived at this period of their existence, and 

 in consequence of their small size, they become difficult to observe.} 

 We have observed the rudiment of the shell at about the seventh day 

 of development. [This is sometimes formed in twenty-four hours in the 

 American species. — Tr.] 



Fertilization will succeed without conforming rigorously to the direc- 

 tions which we have given. The determination Of the sexes, for ex- 

 ample, is not absolutely necessary, for, in operating with a certain num- 

 ber of sexually mature adults, it is certain that both males and females 

 will be found amongst them. The same remark applies to washing 

 the eggs; butwe would insist that it is a useful precaution which has 

 real advantages and facilitates the study of the phenomena of develop- 

 ment. 



* According to the observations made in the laboratory of M. Balhiani by M. Henne- 

 guy, the egg of Ostrea angalata appears to be provided with a micropyle at its point 

 of attachment to the follicle, that is to say, at the extremity of its pedicle. 



t At Verdon we have obtained mobile embryos seven hours after the fecundation, 

 with the water at a temperature of 22° C. (71° F). [Theeggsof the American oyster, 

 with the water at 78° F. to 80° F., will hatch in live hours. — Tit.] 



tThe volume of the larva is about equal to the egg. Now, the egg of (Mrca aniju- 

 lata,if we suppose it to be perfectly Bpherical, and we take the smallest diameter of 

 this sphere, measures 52 millimeters [ T i n th of an inch]; tbe volume is consequently 

 .1073584th of a cubic millimeter. 



For the sake of comparison, measurements of the following species are appended : 



The unisexual oyster of Dackar; diameter of the eg^. 41? millimeters. 



The hermaphroditic oyster of Toulon (Ostrea plkatiila); diameter of the egg, 95- 

 millimeters. 



The common oyster (0. cdulis), hermaphroditic; diameter of the egg, 122 milli- 

 meters. 



