REPRODUCTIVE SYSTEM. 103 



ing' of feathers : in increase of size of mammse, 9 : in pe- 

 riodic discharge of ova, as in Yertebrata (egg-laying reptiles 

 and birds), or accompanied with turgescence of organs of 

 generation (mammalia). Structural changes: as swollen shell 

 for protection of eggs, Unio (mussel), 9 : in development of 

 sculptured capsule for eggs, Argonauta (paper nautilus) 9; 

 in webbed foot and dorsal fold of Salamander, $; in the 

 greater development of thumb of Eana (frog), ; in the 

 growth of antlers, Cervus (deer) $; changes in larynx in 

 certain birds producing song, . 



XV. 



BEPBODUCTIVE SYSTEM, 



THE reproductive system is composed of the organs of gen- 

 eration, and is devoted to the development and retention of 

 the sexual elements. These are sperm cell and ovum. 



SPERM CELL. 



Enclosed within the sperm cell are very generally a num- 

 ber of other cells (daughter cells, vesicles of evolution), within 

 which, in turn, spermatozoa (spermatic filaments) are devel- 

 oped. Each spermatozoon, as a rule, is formed in the same 

 vesicle of evolution with many others, though occasionally 

 singly, as in nematoid Entozoa. Very generally the result 

 of an obscure transformation of the contents of the daughter 

 cell, the spermatozoon may be a modified nucleus, or less 

 frequently the transformed cell itself, as in decapod crus- 

 taceans. 



The forms of spermatozoa are exceedingly various. The 

 principal ones are as follows: slender filament, with ovoidal 

 expansion at one extremity; this expansion oval, acumi- 

 nate, as in man; flat and obtuse in Sciurus (squirrel); sickle- 

 shape, as in Mas (rat); pyriform, as in Canis (dog); verm- 



