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TEXTBOOK OF ZOOLOGY 



tion the cirrus or penis is protruded from the genital pore to enter 

 the genital pore and extend into the uterus 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 fertilization 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 



Fig. 87. — Fission as it occurs in Planaria dorotocephala. 



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, gastru- 

 lation and even later stages before the cocoon ruptures and the small 

 wormlike planarians escape into the water. 



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 



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