332 ESSENTIALS OF ZOOLOGY 



Literus of the other copulant. In this way spermatozoa may be 

 transferred from each animal to the other. Spermatozoa have been 

 found along the oviduct as far as the anterior portion, so fertiliza- 

 tion likely occurs somewhere along this tube. At breeding time 

 zygotes are found in the atrium, and each is surrounded by a large 

 number of yolk cells (nurse cells). Each yolk cell contributes its 

 store of nourishment to the egg cell to which it is attached. From 

 one to several zygotes, surrounded by many thousands of yolk cells, 

 become enclosed in a capsule-like shell secreted by the genital atrium 

 and known as a cocoon. These are expelled from the atrium and 

 each is attached by a stalk to the under sides of submerged stones or 

 vegetation in the water. In the cocoon the embryo passes through 

 cleavage divisions, blastula stage, gastrulation and even later stages 

 before the cocoon ruptures and the small wormlike planarians escape 

 into the water. 



. — Young olanaria 

 \ hacchina 



Icjcj capsule or cocoon 



Fig. 120. — Planarian cocoons with young emerging from one. 



Asexual reproduction by transverse fission occurs quite frequently 

 when the mature animals become slowed down. The individual con- 

 stricts and then divides into anterior and posterior portions, each of 

 which forms the missing parts by rapid cell division. The axial 

 orientation of the tissue is retained ; i.e., an anterior portion develops 

 in the position of the original anterior portion, and a posterior por- 

 tion at the original posterior position. This process is not funda- 

 mentally different from budding in Hydra or strobilization in the 

 Scyphomedusae. 



The retention of the axial orientation during fission has been 

 explained by Dr. Child of Chicago University. The animal pos- 

 sesses a well-defined axial organization in which the ''head" por- 

 tion as usual has the highest metabolic activity of the body. Be- 

 ginning at the anterior there is a gradient of decreasing metabolic 

 activity until a level just posterior to the mouth is reached, and here 

 a sudden increase occurs. Posterior to this the decreasing gradient 



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