OF PLANOCERA ELLIPTICA. 
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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. 
The spermatic particles of Polycelis variabilis 1 have observed, but shall describe 
them in connection with the development of its eggs on another occasion. Those of 
Planocera elliptica have hitherto escaped my notice. Their description would have 
been here in place. 
III. 
THE LAYING OF THE EGGS. 
The spawning period lasts from the middle of May to the middle June, setting aside 
the precocity of some individuals, and the backwardness of some others. 
When the animal lays its eggs, it creeps along the smooth surfaces of the submarine 
bodies, either stones, pieces of wood, or sea-weeds, &c. It leaves behind it a thin band 
of mucosity of the width of its body and in which the eggs are arranged in one layer, 
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