324 THE ANATOMY OF CESTODES. 



the process of division takes place, at least at first, somewhat regularly. 

 The first two divided globules again break up into two pale nucleated 

 balls of equal size, which is of course less than that of the former. 

 When the division has gone further, the segmentation masses collect 

 together, and round themselves off into a spherical body the so- 

 called " mulberry mass " (morula). During these changes the egg has 

 grown to a considerable size, so that the segmented mass alone is 

 larger than the whole mass formerly was. 



One might suppose that this mass of balls changed directly into 

 the later embryo after continuous growth and diminution of its 

 cells, and after the formation of the hooks and shell. But the de- 

 velopment does not take place quite so simply. Van Beneden and 

 Moniez have both observed that in certain species (Tcenia bacillaris, 

 T. expansa) a peripheral layer of cells is first found in the embryonic 

 body, whose elements divide more rapidly, and assume a brighter 

 appearance, than is the case in the other forms. Although at first in 

 close contact with the embryonic body, this cell-layer gradually sepa- 

 rates from it. A cavity is left between the two, at first only in- 

 significant, but rapidly attaining a considerable size, and containing 

 a clear fluid. For a time the cells are distinctly visible around this 

 space, but afterwards they undergo a degenerative process, in conse- 

 quence of which they assume a more or less granular nature, lose 

 their former boundaries, and finally disappear. During these modi- 

 fications the other cell-mass has continued its development. The 

 cells have become smaller, and are so closely packed that they can 

 hardly be recognised as such. In consequence of this the embryonic 

 body has contracted into less bulk. Its boundary has become more 

 distinct, and very soon appears very like a special envelope of homo- 

 geneous nature, which becomes gradually thicker, and raises itself like 

 a mantle from the under layer. Not unfrequently (Tcenia bacillaris, 

 &c.) a second and third envelope appear in addition to this first one. 

 They all originate as excretions from the embryonic body, in the 

 same manner as the so-called egg-shells of Eehinorhynchus.' 1 



Yet it cannot be doubted that the enclosed spherical body is 

 really the embryo. This is evident not only from its developmental 

 history, but perhaps still more clearly from the rapid appearance 

 of the characteristic embryonic hooks. 



It is thus quite inadmissible to compare the firm envelope sur- 

 rounding the embryo in the Tcenice with the egg-shell of the 

 Bothriocephali, which is from the first present in the uterus, and 

 contains not only the embryo but also the yolk. The egg-shell is 

 represented in this case by the outer membranous boundary of the 



1 Fir&t German edition of this work, p. 805. 



