536 STANLEY C. BALL 



place in the several primary entoderm cells. Its function prob- 

 ably has to do with the absorption and digestion by these cells 

 of the vitelline yolk. The behavior of the mitochondrial mass 

 is different from that observed in other animals in that, whereas 

 in most forms it either disappears in the egg stage or, if remaining 

 through cleavage, is restricted to one cell, in P. gemellipara 

 several cells ultimately receive a portion of the original sub- 

 stance. 



3. Three germmal regions are recognizable in the blastula, 

 the mes-ectoderm, primary entoderm and secondary entoderm. 

 These germinal regions do not seem to have been previously 

 recorded in Turbellaria. 



4. The vitellarial yolk-cell membranes and nuclei degenerate 

 after deposition in the capsule ; no temporary epithelium is formed 

 by them outside the yolk mass. 



5. The primary entoderm cells which lie as a cap at the pos- 

 terior end of the embryo, absorb a large part of the vitellarial 

 yolk. When replete their nuclei immediately degenerate. 



6. These cells are then grasped by the amoeboid secondary 

 entoderm cells. After gastrulation the latter completely engulf 

 and digest the former. 



7. During gastrulation the ectoderm cells absorb the free 

 vitellarial yolk left over after the action of the primary entoderm. 



8. The development of the eyes embraces two phases. First 

 the accessory pigment cell arises as a large clear vesicle very 

 early in the differentiation of the embiyo, and becomes gradually 

 filled with melanin granules which crowd the nucleus to one 

 side. Secondly the essential organ, the sight cell, develops 

 externally to the pigment cell, into an invagination of which 

 it pushes, and sends a nerve fiber backward into the anterior 

 horn of the brain. 



9. The sub-dermal muscle fibers develop in this Rhabdocoele, 

 as in Nemerteans, by the differentiation of mes-ectodermic cells 

 from which long slender fibers grow outward beneath the external 

 epithelium. 



10. The intestine consists of large distinct cells whose free 

 ends extend mto the lumen. It arises after birth by the inter- 



