722 LIFE : OUTLINES OF GENERAL BIOLOGY 



may be enclosed in a packet or spermatophore, which breaks 

 up after being deposited among the eggs; while others, as in some 

 of the highest members of the class, have complex explosive sperma- 

 tozoa which bm-st violently when their outstanding processes touch 

 and adhere to an eg§, the result being that the nucleus and a 

 centrosome are forced through the egg-membrane. In Ostracods 

 the sperms do not become mobile until they pass into the female 

 genital duct, and in some of these water-fleas the spermatozoa 



Fig. iio. 



A Spermatozoon of a Newt. After Ballowitz. i, apical spur; 2, nucleus, com- 

 posing most of the head; 3, tail with vibratile fringe (4),. and end piece (5). 



are gigantic, even g inch in length, three or four times longer than 

 the whole tiny animal! We need give no further illustrations of 

 variety among spermatozoa. 



SPERMATOGENESIS AND OOGENESIS.— In sponges the germ- 

 cells arise in the middle stratum or mesogloea of the body, which has 

 not yet acquired organs of any kind. In Coelentera the germ-cells arise 

 in connection with either the inner or the outer layer (ectoderm or 

 endoderm), and in some cases there are well-defined reproductive 

 organs or gonads, such as are conspicuous in the common jellyfish, 

 AureHa, which swims in all the seven seas. In other animals the 



