THE DEVELOPMENT OP VERTEBRATA. 213 



developed out of a large, yolk-laden egg, goes through so 

 much of its development within the egg-membrane, that 

 when it at last escapes it differs from its parents in only 

 minor characters. Such a developing individual within the 

 vitelline membrane is called an embryo. 



2. The Spermatozoon is almost always smaller than 

 the ovum, even when that is quite small, and as it never 

 contains yolk the disproportion between its size and that of 

 the ovum may be enormous. The spermatozoon (fig. 158[5]) 

 is usually little more than a nucleus (male pronucleus) 

 with a long vibrntile flagcllum (resembling a long cilium) 

 .attached, by means of which it swims actively about until 

 it comes near an ovum. 



3. Maturation of the Ovum. Before fertilization can 

 take place, certain changes happen in the germinal vesicle 

 (nucleus of the ovum). Its bounding membrane dis- 

 appears, much of its substance appears to mingle with the 

 general protoplasm, and the chromatin that remains (far 

 smaller in bulk than the original nucleus) divides by karyo- 

 kinesis into two nuclei : one of these travels to the surface 

 and becomes separated from the ovum, taking a little 

 protoplasm with it. Evidently we have here a peculiar 

 case of cell-division, in which the two cells are extremely 

 unequal in size, though their nuclei are equal. The small 

 cell thus cut off is called the first polar body. The residual 

 nucleus of the ovum again divides in the same way, and a 

 second polar body is extruded. What remains of the 

 original germinal vesicle is now only equal in bulk to the 

 male pronucleus of the spermatozoon : it is called the 

 female pronucleus. These changes constitute the matura- 

 tion of the ovum. 



4. Fertilization. Before the ovum thus matured can 

 develop into a new individual, it is necessary that new 

 nuclear material should be introduced into it. This is 

 effected by the union of a spermatozoon with the ovum. 

 The male pronucleus enters the ovum, and unites with the 

 female pronucleus in a very intimate manner to form a new 



