PLATTELMINTHES. 157 



segments, which appear to give rise to the epiblast, increase in number 

 by division and gradually envelop the large segments '; so that an 

 epibolic invagination cleaxly takes place. Between the small and the 

 large cells is a small segmentation cavity, fig. 86 A and B. At the time 

 when twelve epiblast cells are present, each of the four large cells 

 divides into two unequal parts (Hallez), fig. 86 A. In this way four 

 large (hy) and four small cells (in) 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 epiblast, 



FIG. 86. SECTION'S THROUGH THE OVUM OF LEPTOPLANA TBEMELLAKIS IN THREE STAGES 



OF DEVELOPMENT. (After Hallez.) 



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



the mesoblast cells gradually travel towards the formative 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, TO). 



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 the nearly 

 closed blastoporic area to four small cells. In connection with these 

 cells a complete hypoblastic wall becomes subsequently established, 

 which encloses the original large hypoblast cells. The latter then 

 become resolved into a vitelline mass. 



From a comparison with other types it may be regarded as pro- 

 bable that the enteric wall originates by a process of continuous 

 budding off of small cells from the large cells, which commences 

 with the formation of the four cells above mentioned. 



The blastopore becomes nearly obliterated, but whether it gives 

 rise to the mouth, which is formed in the same place, has not been 

 determined. In front of the mouth a small and very transitory 

 rudiment of an upper lip makes its appearance. The protrusible 

 pharynx is stated by Hallez to arise as an hypoblastic bud, while its 

 sheath has an epiblastic origin. Two pairs of eyes and the supra- 

 cesophageal ganglia also become early developed. 



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. 



