ONTOGENY 



239 



Fertilization. — If mature sea urchin or starfish eggs are placed 

 in a shallow glass dish under a microscope and a drop of sea water 

 containing ripe sperm from a male is added, one can readily see 

 that almost at once each egg is surrounded by a vibrating fringe 

 of spermatozoa, each of which seems to be doing its utmost to gain 

 entrance into the egg (Fig. 147). The spermatozoa reach the egg 



Fig. 146. — Types of spermatozoa. A, salamander. {After Ballowitz.) B, 

 annelid (Nereis). (After Lillie.) C, guinea pig. (After M eves.) D, bird (Phyl- 

 lopneuste). (After Ballowitz.) E, sturgeon. (After Ballowitz.) F , bat. (After 

 Ballowitz.) G, turbellarian (Castrada). (After Luther.) H, crustacean (Pin- 

 notheres). (After Koltzoff.) I, crustacean (Homarus). (After Herrick.) J, 

 nematode (^.scan's). (After Scheben.) a, apical body; n, nucleus; r, refractive 

 body. (From Sharp, Introduction to Cytology.) 



by swimming movements executed by whiplike contractions of 

 the tail, but these mechanical movements do not completely 

 account for the results. Thus it has been shown that ripe eggs 

 of a number of marine forms, when placed in sea water, 

 give off a substance called fertilizin, which forms a necessary 

 link in the fertilization process; for if this substance is removed 

 from the eggs by repeated washings in sea water, the washed eggs 

 are not fertilized when brought in contact with normal active 



