310 EMBRYONIC DEVELOPMENT 



future being ? and if so, which part is it ? Before any facts, answering the above 

 queries, shall be experimentally demonstrated, no theory will ever have a chance of 

 being adopted. The egg of Ascidia and that of Planaria are easily watched throughout 

 their entire mass, and should a spermatozoon or spermatic particles penetrate into 

 the vitellus, and constitute any part or portion of the future being, such a phe- 

 nomenon could not escape the eye of a careful observer. If the union of the sper- 

 matic substance with the vitelline substance is a necessary act in the procreation of the 

 embryo, that union is molecular and beyond the reach of observation with the optical 

 apparatus now at our command : when the spermatic particles lose their activity and 

 disappear to the observer's eye, they disintegrate into elementary molecules, in all 

 probability minute cells, which enter the egg by endosmosis. That substance, in 

 mixing with the vitelline substance, operates as a catalytic. For, no sooner has the 

 phenomenon of the fecundation taken place, that the vitellus enters into the phases 

 of the division, which is a mere kneading of the vitelline or embryonic substance. 

 That labor performed, the embryo issues forth out of the most homogeneous cellular 

 substance, the substance of the vitellus. 



Such, it appears to me, is the probable union, if union there is, of the elementary 

 substance furnished by either sex in the procreation of a new being. The spermatic 

 substance in its most intimate structure, does not differ materially from that of the 

 vitellus ; in both cases we may have minute cells, nucleated or not, transparent or 

 semitransparent, very similar in general appearance. 



What has just been said of the material act of the fecundation, was not observed 

 upon the eggs of Planocera elliptica, the subject of this memoir, but in a species of 

 Ascidia. When I shall investigate, for another memoir, the structure and functions 

 of the sexual organs in Nemertians and Planarians, an opportunity may be afforded me 

 to witness the fecundation in these two groups. 



Among Planarians there are species in which every individual is provided with both 

 sexes ; there are others where we find each sex, represented by a different indi- 

 vidual ; accordingly the reciprocal action of these organs must take place in various 

 ways. 



In Planocera elliptica each individual posesses ovaries and spermaries, that is to 

 say is an hermaphrodite. Whether two individuals are required to consummate the 

 act of fecundation, as in the snails and slugs, I am not yet prepared to say. 



In many species there are, properly speaking, no ovaries ; the eggs are formed 

 all around the ramifications of the stomach, whence passing through an oviduct into 

 a pouch, supposed by anatomists to be a coital pouch. I have no settled opinion in re- 

 gard to this organ, but I am satisfied that when in it, the eggs assume the last feature 

 of intra-ovarian life. 



The fecundation may either take place before or after the eggs are laid, according 

 to their genera or species. 



