PHYLUM PLATYHELMINTHES 



171 



are stored in advance of copulation. Here they become organized 

 into pockets known as spermatophores. The ova mature in the 

 ovaries, pass down the oviducts where yolk cells or nurse cells are 

 added by the yolk glands, through the vagina to the genital atrium, 

 and probably from here to the uterus or seminal receptacle where 



Young planana 

 batchinq 



tq(j capsule or cocoon 



Fig. 85. — Planarian cocoons and the young hatching. 



Vitelline cells 



Wandering 

 cells 



Endodenn 



Provisional 

 pharynx 



Ectoderm 



Wandering 

 " cells 



Endoderm 

 -Primitive 



gut 

 Wandering 

 — cells 



Vitelline 

 ceUs 



Mouth 



Fig. 86. — Development of Planaria lactea. 1, E!gg surrounded by yolk; 2, four 

 blastomeres from segmented egg ; S, later stage ; if, still later, after blastomeres 

 have differentiated into ectoderm, endoderm, a provisional pharynx, and wandering 

 cells; 5, cellular differentiation more advanced; 6. embryo becomes flattened and 

 ovoid. (From Hegner, College Zoology, published by The Macmillan Company, 

 after Lankester after Hallez. ) 



they are thought to be stored. Cross-fertilization is practiced by these 

 animals. Planarians have been observed to copulate with an apparent 

 exchange of spermatozoa in the form of spermatophores. In copula- 



