Embryology. 



127 



observers to take place also from the male germ-cell, 

 or spermatozoon, at or about the close of its develop- 

 ment. The theories to which these facts have given 

 rise will be considered in future chapters on Heredity. 



Turning now to the mechanism of fertilization, the 

 diagrams (Figs. 34, 2tS) represent what happens in 

 the case of star-fish. 



The sperm-cell, or spermatozoon, is seen in the act 

 of penetrating the ovum. In the first figure it has 

 already pierced the mucilaginous coat of the ovum, 



?b»,^'"^ 



Fig. 35. — Fertilization of the ovum of a star-fish. (From the Encycl. 

 Brit, after Fol.) A, spermatozoa in the mucilaginous coat of the 

 ovum ; a prominence is rising from the surface of the ovum towards 

 a spermatozoon; B, they have nlmost met; C, they have met ; D, 

 the spermatozoon enters the ovum through a distinct opening; H, 

 the entire ovum, showing extruded polar bodies on its upper surface, 

 and the moving together of the male and female pronuclei ; E, F, G, 

 meeting and coalescence of the pronuclei. 



the limit of which is represented by a line through 

 which the tail of the spermatozoon is pa.'^sing : the 

 head of the spermatozoon is just entering the ovum 

 proper. It may be noted that, in the case of many 

 animals, the general protoplasm of the ovum becomes 

 aware, so to speak, of the approach of a spermatozoon, 

 and sends up a process to meet it. (Fig. '3,^, A, B, C.) 

 Several — or even many — spermatozoa may thus enter 

 the coat of the ovum ; but normally only one proceeds 

 further, or right into the substance of the ovum, for the 



