26 The Microscope. 



but as yet no vitelline membrane. The spermatozoa become 

 entangled in this mucus, and hence travel much more slowly 

 here than in the sea- water. Almost immediately the proto- 

 plasm sends out a long and slender process to meet the nearest 

 spermatozoon ; or if there be two or more equally near, a cor- 

 responding number of protoplasmic prolongations are found, as 

 shown in Fig. 4. Soon the process and the spermatozoon meet, 



Fig. 4. — An egg sending out a protoplasmic filament to meet a 

 spermatozoon. 



Fig. 5. — Entrance of a spermatozoon into an egg. /, Female pronucleus. 

 m, Male pronucleus, s, Remnant of spermatozoon. 



and then the former begins to retreat to the surface of the egg, 

 drawing the latter with it (Fig. 5.) Now the egg secretes the 

 vitelline membrane, and no attention is paid to other spermat- 

 ozoa. Gradually the spermatozoon sinks into the egg, and as 

 its head penetrates the yolk there begins the formation of a 

 clear space like the female pronucleus, which from its origin is 

 known as the male pronucleus. 



These two pronuclei now approach each other, and finally 

 unite to form the nucleus of segmentation. In most cases but 

 a single spermatozoon is needed to fertilize an egg; indeed, as 

 we shall see further on, the entrance of more than one of these 

 organisms being in some cases decidedly disadvantageous. In 

 other cases several are required to perfect the operation. 



In eggs with a hard chorion there is frequently an opening 

 known as the micropyle, by which the spermatozoon ob- 

 tains entrance to the egg proper; in others there is nothing 

 of the kind. In our friends, the insects, one end of the egg 

 usually bears a peculiar and complicated apparatus known as 

 the micropylar cap, which will be described in somewhat of 

 detail when treating of the development of those forms. 



c. Segmentation. 

 After the union of the male and female pronuclei to form 



