REPRODUCTIVE SYSTEM. 103 



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

 riodic discharge of ova, as in Vertebrata (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), ?; in development of 

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

 in webbed foot and dorsal fold of Salamander, 1} in the 

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

 growth of antlers, Ccrvus (deer) ; changes in larynx in 

 certain birds producing song, $ . 



XV. 



BEPEODUCTIVE 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 occasional I} 7 

 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 Cams (dog); verm- 



