TURBELLARIA. 



time, and adhere together so as to form masses not unlike the 

 spawn of nudibranchiate Molluscs. 



Within the egg-capsule the ovum floats freely and undergoes 

 a segmentation similar in many respects to the characteristic 

 molluscan type. The ovum divides into two, and then into four 

 parts, from each of which a small segment is then separated off. 

 The four small segments, which appear to give rise to the epi- 

 blast, increase in number by division and gradually envelop the 

 large segments 1 ; so that an epibolic invagination clearly takes 

 place. Between the small and the large cells is a small segmen- 

 tation cavity, fig. 86 A and B. At the time when twelve epiblast 

 cells are present, each of the four large cells divides into two un- 

 equal parts (Hallez), fig. 86 A. In this way four large (/if) and 

 four small cells (m) are formed. The latter are placed at the 

 opposite pole of the ovum to the epiblast cells, and give rise to 

 the mesoblast, while the four large cells remain as the hypoblast. 



In the course of the enclosure of the hypoblast cells by the 



FIG. 86. SECTIONS THROUGH THE OVUM OF LEPTOPLANA TREMELLARIS IN THREE 

 STAGES OF DEVELOPMENT. (After Hallez.) 



ep. epiblast ; m. mesoblast ; hy. yolk cells (hypoblast) ; bl. blastopore. 



epiblast, the mesoblast cells gradually travel towards the forma- 

 tive pole (fig. 86 B). In the process they become first of all 

 divided so as to form four linear streaks, and finally unite into a 

 continuous layer between the epiblast and hypoblast, which 

 obliterates the segmentation cavity (fig. 86 C, m). 



Before the completion of the epibole a closely packed layer 

 of fine cilia appears, which causes a rotation of the embryo within 

 the egg-capsule. During the above changes a fifth hypoblast 

 cell is formed by the division of one of those already present ; 

 and at a later period four of the hypoblast cells give rise within 



1 It is probable, though it has not been observed, that the growth of the layer of 

 small cells is assisted by the formation of fresh cells from the hypoblast spheres. 



