SEX-CELLS 



565 



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 



alb 



CLlb 



FIG. 148. Semi-diagrammatic view of the egg of the fowl at the beginning of 



incubation. 



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

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

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

 membrane ; sh.m. /, sk.m. 2, its two layers separated and enclosing 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. Fertilisation has also been described 

 as the conjugation or fusion of two gametes, the ovum and 

 sperm (compare p. 197). We have now to consider in 

 greater detail what is known as to the precise mode of 



