174 



EMBRYOLOGY 



sule is drawn out into a stalk, by means of which it adheres 

 to fixed objects (Fig. 86), in much the same waj^ as in 

 the fresh-water Dendrocceles. In each capsule there is 

 usually found only one egg-cell, in rare cases {Prostomum 

 Steenstrupii) two of them. As in the fresh- water Dendro- 

 cceles, the egg-cells occupy only a small part of the capsule, 

 the remaining space being filled with yolk-cells (Fig. ^Q). 



In spite of the presence of the yolk- 

 cells, development proceeds in a manner 

 similar to that of the Polycladida. Per- 

 haps when the development of the 

 Ehahdoccelidcs becomes more accurately 

 known intermediate conditions will be 

 found here, which will explain the 

 aberrant condition of the Tricladida. 



After the expulsion of the polar 

 globules and fertilization, the egg di- 

 vides first into two, then into four, 

 blastomeres of equal size. Four 

 smaller ones are constricted off from 

 these (Salensky). The further cleav- 

 age processes could not be observed 

 by Hallez ; their result, however, is 

 an epibolic gastrula, which entirely 

 resembles that of the Polycladida. The 

 ectoderm becomes covered with cilia, and the embryo floats 

 in the mass of yolk-cells. It is therefore equivalent to 

 a larva, which, however, does not attain to a wholly free 

 life, just as the larvae of the Gnathohdellidoi and Oligochcetce 

 live within the cocoon, and are nourished by the albumen 

 occurring in it. 



In a later stage of the embryo the entoderm is seen to be 

 arranged in a continuous layer. Its cavity becomes connected 

 with the outer world by means of the pharynx. It appears 

 as if it were, as in the Tricladida, of entodermal nature. The 

 yolk-cells are conveyed by means of it into the intestine. 

 Yet the pharynx of the Rhahdocoelidce, contrary to that of the 

 Tricladida, at once reaches its permanent form. The 

 primitively spherical embryo by becoming elongated and 



Fig. 8G.— Stalked egg 

 capsule of Prostomum 

 Steenstrupii with two egg- 

 cells (Ez) and surrounding 

 yolk-cells (after Hallez), 



