Embryology. 127 
observers to take place also from the male germ-ccll, 
or spermatozoon, at or about the close of z¢s 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, 35) 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, 
Fic. 35.—Fertilization of the ovum of a star-fish. (From the Zvcycl, 
Brit. after Kol.) 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 almost mct; C, they have met; D, 
the spermatozodn 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 fimale pronuclea; E, F, G, 
meeting and coalescence of the pronuclei. 
the limit of which is represented by a line through 
which the tail of the spermatozoén is passing: 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. 35, 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 
