132 



DICYEMID^E. 



centre. In these spheres a cortical layer becomes differentiated, 

 which gradually increases in thickness and gives rise to the body 

 of a cell, the nucleus and nucleolus of which are respectively 

 formed from the inner part of the original sphere and the minute 

 central body. These germs can originate in all parts of the 

 hypoblast cell and are frequently very numerous. 



The germ when completely formed undergoes a segmentation 

 very similar to that of an ordinary ovum. It divides first into 

 two and then into four approximately equal segments. Of the 

 four segments one, however, remains passive for the remainder 

 of the development. The other three divide and arrange them- 

 selves so as partially to enclose in a cup-like fashion the passive 

 cell (fig. 6 1 A). The six cells resulting 

 from their division again divide, giving 

 rise to twelve cells, which nearly enclose 

 the passive cell, leaving only a small 

 aperture at one point. The whole pro- 

 cess by which the central cell becomes 

 enclosed is, as E. van Beneden points 

 out, identical with a gastrula formation 

 by epibole, and the space where the 

 central cell is left uncovered is the blas- 

 topore. The central cell itself gives 

 origin to the hypoblast cell of the 

 adult, and the peripheral cells to the epiblast. 



By this time the embryo has assumed an oval form, and the 

 blastopore is situated at the pole of the long axis of the oval 

 where the cephalic enlargement is eventually formed. 



The subsequent development consists mainly in the closure 

 of the blastopore, and an increase in the number of the epiblast 

 cells. Before the development is completed, and while the 

 embryo is still in the body of the parent, two germs, destined 

 themselves to give rise to fresh embryos, appear in the hypoblast 

 cell, one on each side of the nucleus (fig. 61 B). The embryo 

 continues to elongate, while the anterior cells become converted 

 into the polar cells. Cilia appear simultaneously over the 

 general surface, and the embryo makes its way out of the body 

 of the parent, usually at the cephalic pole, and becomes itself 

 parasitic in the renal organ of the host in which it finds itself. 



A. 



FIG. 61. A. GASTRULA 



STAGE OF DlCYEMA TYPUS. 



B. VERMIFORM EMBRYO OF 

 DICYEMA TYPUS. (From 

 Gegenbaur, after E. van 

 Beneden.) 



