154 



COLLEGE ZOOLOGY 



penis ( $ ), a muscular organ which enters the genital cloaca. A 

 seminal vesicle lies at the base of the penis, also a number of uni- 

 cellular, prostate glands. Spermatozoa originate in the testes, 

 and pass, by way of the vasa deferentia, into the seminal vesicle, 

 where they remain until needed for fertilization. 



FIG. 101. Development of Planaria lactea. i, egg (0) surrounded by 

 yolk (v). 2, four blastomeres (&/) from segmented egg. ' 3, later stage; blas- 

 tomeres (bl) more numerous. 4, much later stage; blastomeres differentiated 

 into ectoderm (ep), entoderm (hy), a provisional pharynx (ph), and wandering 

 cells (w~). 5, cellular differentiation more advanced; ep, ectoderm; ent, 

 primitive gut; hy, entoderm; ph, pharynx. 6, embryo changes shape to a 

 flattened ovoid; ent, primitive gut; m, mouth; ph, pharynx. (From Lan- 

 kester's Treatise, after Hallez.) 



The female reproductive organs comprise two ovaries (g), two 

 long oviducts (od) with many yolk-glands (V) entering them, 

 a vagina ( $ ) which opens into the genital cloaca, and the uterus 

 which is also connected with this cavity. The eggs originate in 

 the ovary, pass down the oviduct, collecting yolk from the yolk- 

 glands on the way, and finally reach the uterus. Here fertilizer 



