OF PLANOCERA ELLIPTICA. 
25 
§ 2. The organic process which is performed throughout the successive periods 
in the history of the egg of Planocera elliptica, is the most simple of the animal 
economy : The egg commences as a cell; in it, neuclei are formed, the points of 
departure of the vitelline cells proper, which multiply by exogenesis. These vitel- 
line cells, simple and homogeneous at first, enter into a process of diversification , 
from the instant that the organic vitelline sphere has received the impulse of de- 
velopment, by which a new individual is called into existence. Once started, this 
diversification of the constituent organic cells proceeds onwards, until the new being 
has completed its growth; when all the organs, which its sphere of action involved, 
have reached that degree of fulness or completion, necessary to fulfil the end con- 
templated in the plan of structure to which the animal belongs. 
XVI. 
CONCLUSION. 
§ 1. The investigations which I have traced upon Planarians have led me to their re- 
moval from the class of Worms, where they had ranked hitherto, into the division of 
mollusca, and more particularly into the class of Gasteropoda.* The embryological 
grounds for mv so doing are the following t 
The embryogeny of Gasteropod molluscs, and more particularly of Nudibranchiata 
has such a striking resemblance with that of the Planarians which I have examined, 
< 
that any one familiar with the subject will acknowledge its evidence. Thus the divi- 
sion of the vitellus in Polycelis variabilis , as observed by me several years ago, although 
not published yet, seems almost an exact copy of the same phenomenon in Acteon vi~ 
ridis of the coast of France : when the yolk is divided into four spheres, four smaller 
ones will appear opposite, and then the latter will remain stationary whilst the former 
will follow out the process of the division. 
In Planocera elliptica the division of the yolk does not differ apparently from the 
same phenomenon in Acteon clilorolicus of New England, and likewise in several spe- 
cies of Eolis and Doris, as well as Triton. There is the most striking resemblance 
in that respect between Planocera ellipiica and Eolis gymnota , in the cases in which 
the yolk divides into three spheres instead of four. 
The vitellus, as a whole, transforms itself into an embryo ; there being no embryonic 
layer distinct from any other portion of its mass. 
* Essai on the Classification of Nemertes andPlanaria, &c. — Amer, Jour, of Sc. and Arts, 2d Series, si. 1851, 
41, and Proc. Amer. Assoc. Adv. of Sc., third meeting held at New-IIaven in August, 1850, Washington, 1851, 
258. 
