548 



SEX-CELLS 



words, that multicellular animals, however large and complex 

 they may be in their adult condition, originate as unicellular 

 bodies of microscopic size ; and the same is the case with 

 plants. 



Spermatogenesis and Oogenesis. In the preceding 

 chapters it has more than once been stated that sperms 



sh.m. 



'\G. 138. — Semi-diagrammatic view of the egg of the fowl at the beginning cf 

 incubation. 

 a. air-space ; alb. dense layer of albumen ; alb' . more fluid albumen ; /'/. blasto- 

 derm ; ch. chalaza, a twisted cord of the dense layet of albumen at either end 

 of the egg, formed as the latter rotates down the oviduct ; sh. shell ; sh. in. shell- 

 membrane ; sh.Ttt.I, sh.in.2, its two layers sexlarated to enclose air-cavity. (From 

 Parker and Haswell's Zoology^ after^Marshall, slightly altered.) 



arise from ordinary undifferentiated cells in the spermary, 

 and that ova are produced by the enlargement of similar 

 cells in the ovary. Fertilization has also been described 

 as the conjugation or fusion of ovum and sperm (compare 



