PLATYHELMINTHES 



111 



cells lie in the furrows between the large cells of the fourth quartette, 

 and each of them divides again into two equal cells, the lower of which 

 2' 222 , etc. reaches and touches the corresponding macromere, thus 

 completing the covering of the egg with ectoderm. 2a n , etc. on the 

 other hand, which had already budded off 2a 1 ' 2 as a smaller external 

 cell, repeats the process, giving rise to 2a 112 externally, and 2a m 

 internally. This last cell, 2a m , with the corresponding cells formed 



mes 



.eel 



FIG. 86. Three longitudinal sections through developing embryos of Planocera inquilina. 



(After Surface.) 



Letters as in Fig. 84. In addition, y, yolk spheres resulting from the disintegration of the cells, 

 4a, 46, and 4c ; stom, stomodaeum. A, the endoderm forms a solid mass of cells in which the gut- 

 cavity is just beginning to appear. B, later stage, the gut-cavity has appeared. C, longitudinal 

 section of Miiller's larva just after its escape from the egg-capsule. 



by the other members of the quartette 2& 111 , 2c m , 2d ni , give rise 

 to the mesectoderm, lying at the lower pole of the egg, which almost 

 certainly forms the musculature of the stomodaeum which serves as 

 larval pharynx. 



The third quartette of micromeres undergoes only equal divisions 

 and all the daughters remain at the surface and are incorporated 

 in the ectoderm. Beyond this stage Surface could not trace the 

 ancestry of the cells of the embryo with any certainty. 



