CHAP, iv.] OF ANIMAL GENERATION. 327 



completing themselves little by little. They axe then vitelline 

 cells, the agglomeration of which gives to the ovular surface the 

 appearance of a mulberry (Figs. 49 and 50). 



Soon after their appearance the vitelline cells displace them- 

 selves ; they come and fix themselves on the internal face of the 

 ovular membrane (vitelline), and thus form a membrane, limiting 

 a hollow sphere full of an albuminous liquid. This membrane 

 has been called blastoderm, and the 

 cells which compose it become the 

 llastodermic cells (Fig. 50). It is 

 at their expense that the embryon 

 forms itself. 



The vitelline fractionment is a 

 general phase of ovular evolution ; 

 but it is not always accomplished 

 uniformly. In birds, in fishes, in 

 testaceous reptiles, in the cephalo- 

 podal mollusks, it is partial ; a por- 

 tion only of the vitellus takes part 



therein. Then the blastodermic Orum of rabbit taken in the uterus. 



, . , a, pellucid zone; ft, blastodermic vesi 



membrane, instead OI forming a c ie formed of hexagonal cells, in sym- 



, TT -, -, . , metrical conjunction like the bricks 



noilOW spnere, becomes a Simple f f a pavement] ; c, accumulation of 



spherical calotte, and the portion ie lc < 



of the vitellus not employed plays merely the part of a nutritive 



substance. 



i 



In the ovum of insects, of the arachnida, of certain crusta- 

 ceans, there is produced, instead of a regular series of biparti- 

 tions, an irregular cleaving, whence results the formation of 

 unequal fragments. But these fragments end by grouping 

 themselves into polyhedrical masses. 



Whatever besides may be the form and the extent of the blas- 

 todermic membrane, it is not long in unfolding into two super- 

 posed membranes. This division is especially perceptible at 

 the point of the blastoderm, whither go to form themselves the 

 first lineaments of the embryon. The external blastodermic 



