M. Prevost on the Generation of Animals. 335 



some time in investigating the phenomena of generation in 

 various classes of animals ; and the papers noted above, of 

 which he has had the goodness to transmit copies to us, were 

 read at various periods before the Physical and Natural His- 

 tory Society of Geneva. The object of these investigations 

 is to support the opinion, that, among vertebrated animals, the 

 developement of the embryo does not take place till after con- 

 tact between the cicatricula of the female ovaries and the sper- 

 matic animalcules of the male, which they conceive to be the 

 chief agents in effecting fecundation. The first memoir, on 

 the generation of the Myoi Pictorum, shows that among mol- 

 luscous animals the same law is followed. " If, towards the 

 spring," says M. Prevost, " we examine the organs of generation 

 in some individuals of this species, we are struck at the first 

 glance with the different products which they emit. While 

 we find in some individuals a true ovary, and ova in abun- 

 dance, in others the analogous organs, and similarly placed, 

 contain nothing but a thick liquid of a milky colour, which 

 under the microscope appears to be crowded with animalcules 

 in motion. These marked differences are neither the result 

 of chance, nor of a subsequent change in the condition of the 

 ovary. The Mi/ce in which ova are found present no trace of 

 the thick and milky fluid ; and, on the contrary, those which 

 possess this liquid produce no ova." M. Prevost, after de- 

 scribing the state of the parts in these different individuals at 

 different periods, by the aid of the microscope, comes to the 

 conclusion, 1. That the white liquid in the organs of genera- 

 tion in one class of individuals has so much analogy with the 

 spermatic apparatus of vertebrated animals that it may be con- 

 sidered as performing the same functions ; and, 2. That 

 since the seminal fluid and ova are never found in the same 

 individual, it may be concluded, though contrary to the gene- 

 rally received opinion, that this genus of animals have the 

 sexes in separate individuals. M. Prevost confirms this opi- 

 nion by direct experiment ; and the memoir is illustrated by 

 an engraving of the different appearances. 



The next memoir is on the generation of the Helix palustris ; 

 and here, though the animals are hermaphrodite, or possess 

 both male and female organs of generation, M. Prevost shows 



