534 STANLEY C. BALL 



posterior migration it has becorae arranged many rows thick 

 between the ectoderm and a few large clear cells in the center. 

 The latter are believed to belong to the primary entoderm which 

 ordinarily absorbs the yolk; in this case it is probable that no 

 food was enclosed with the egg in the capside. 



The behavior of the entoderm cells in P.gemellipara is compa- 

 rable to that observed in 1884 by Lang in the Polyclad Discocoelis. 

 Since the egg of this worm contains a rich supply of yolk material, 

 none is added in the form of vitellarial cells. During segmenta- 

 tion an upper and a lower quartet of entoderm cells arise whose 

 derivatives later form the wall of the alimentary canal. These 

 entoderm cells have a more or less amoeboid character, sending 

 out protoplasmic processes over the yolk spheres. The latter 

 were previously split off from the yolk-filled middle entoderm 

 which takes no part in the formation of any organ. 



In 1907 Surface described the development of the alimentary 

 canal in Planocera inquilina. He found that practically all 

 of its tissue arises from two large entoblast cells. By division 

 they form two sorts. One group containing the yolk is pushed 

 into the interior of the embryo where the cells break down and 

 the yolk granules flow together into a fluid mass of large spheres. 

 Meanwhile the lower entoblast cells divide rapidly, thus giving 

 rise to a large group of cells. A cleft, the first indication of the 

 intestinal lumen, now occurs in this entodermic mass, and cell 

 outlines become distinct about the nuclei . The important feature 

 to be noted in relation to the behavior of the entoderm of P. 

 gemellipara is that Surface observed these cells to become amoe- 

 boid and to spread out on the surface of the large yolk spheres 

 which are thus absorbed. 



Now it is only a short step from the condition described by 

 Surface in a Polyclad to that which I have observed in the 

 Rhabdocoele, Paravortex. Here two entoderm elements — the 

 large macromeres of text figure 14 — give rise by division to the 

 primary and secondary entoderm cells. The former, since 

 they contain no yolk of their own, absorb it from the vitellarial 

 substance. Like the upper yolk cells in Planocera, their nuclei 

 degenerate and their membranes break down. But this latter 



