178 EMBRYOLOGV 



II. TREMATODA. 



The egg of Trematodes is a product of tlie ovarium and the 

 vitellaria. The latter supply to each egg a num.ber of cells, 

 which surround the egg-cell, and in the course of develop- 

 ment are consumed by the embryo. [The conditions are, 

 therefore, much the same as in the Turbellaria, and what we 

 call the egg in the Trematodes is a composite structure 

 similar to the cocoon of the Triclads and Rhabdocoeles, save 

 that there is only a single egg-cell and that the number of 

 yolk-cells is much smaller. — K.] The embryo leaves the egg 

 usually at a stage of development which is still far removed 

 from the organization uf tlie parent. Before it reaches this 

 it still has to undergo a complicated process of development. 



I. DlSTOillD^. 



The embryonic development has been the most 

 elaborately treated in tlie investigations of Schauinsland 

 (No. 8). In Distomum tereticoUe the egg-cell lies at that pole 

 of the egg which is marked by the operculum of the egg- 

 shell (Fig. 87 A). The remaining part of the egg is formed 

 by the yolk-cells, which still show their cellular structure, 

 but gradually undei'go degeneration. The egg-cell divides 

 into two cells, four cells, etc, until the germ extends over a 

 large part of the entire egg (Fig. 87 JJ and C). At the apex 

 of the embryonic mass a cell is soon distinguished from 

 the rest by its losing its spherical shape and covering the 

 uppermost cells like a kind of cap (Fig. 87 C, Kz). It soon 

 divides into two cells, which grow downward, becoming in 

 this way attenuated iuto a thin membrane. Still other cells 

 take part in the formation of this delicate enveloping mem- 

 brane as soon as the nutritive yolk is entirely displaced by 

 the cleavage cells (Fig. 87 D and JE). At this stage the 

 germ is a solid mass of cells surrounded by the enveloping 

 membrane, which is separated by a narrow space from the 

 cell-mass (Fig. 87 E). Under the enveloping membrane is 

 developed around the entire circumference of the embryo a 

 layer of flat cells, which Sghauinsland looks upon as 



