548 



SEX-CELT,S 



words, that niulticenular 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 ilie preceding 

 chapters it has more than once been stated that sperms 



I ^8.— Sf-ini-iliaer.'imm.'i 



:iv ,,r Ihc csg .,f ihr r.iul al ill.- Iic!;iiiiiiiii; .f 

 inculi.itioii. 



rt. .-lir-space ; alh. dense layer of albumen ; all'' . more (tllid albumen ; /'/. l)laslo- 

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

 of the egg, formed as the latter rotates down the o\iduct ; ,v//. shell ^ sli. iit. shell- 

 membrane ; sh.iJt.l^ s/t.)ii.2, its two layers separated to enclose air-i-a\ it>-. (from 

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



arise from ordinary undifferentiated cells in the spermar\', 

 and that ova are produced bv the enlargement of similar 

 cells in the ovary. Fertilization has also been described 

 as the conjugation or fusion ot ovum and sjierm (compare 



