28 



ZOOLOGY. 



45. The Fertilized Ovum the Starting Point. In speak- 

 ing of the development of the adult animal from the simpler 

 condition of the single cell it is necessary to remember that 

 this cell, which has the power of giving rise to a complex in- 

 dividual and is called a fertilized ovum, has a history that is 

 very important. The fertilized ovum represents the union of 

 two distinct cells, known as germ or sexual cells, which are 

 ordinarily quite different in appearance and produced by dif- 

 ferent kinds of individuals, males and females. Both classes 

 of cells may be produced by the same individual. This union 

 does not produce a double cell, but the parts of each seem to 

 fuse with those of the other in a very complete way. 



46. The Ovum. The female germ cell is known as the 

 ovum, and is typically a spherical cell with abundant nourish- 



FIG. 8. Types of ova. A, primitive amoeboid ovum of Sponge; B, semi-diagram- 

 matic figure of spherical ovum of Sea-urchin in which the yolk is uniformly distributed; 

 C, figure of a spherical ovum (such as may be found in some Worms or in the Frog) in 

 which the yoke tends to collect at one pole, p.p., and the nucleus and protoplasm at the 

 other, a.p.; m, micropyle; nu, germinal vesicle (nucleus); n, germinal spot (nucleolus) ; 

 y, yolk spheres. 



Questions on the figure. What are the points of agreement in these 

 three ova? The chief points of contrast? What is the function of the 

 micropyle? Is a micropyle always present in ova? Why are the poles of 

 the ovum appropriately called active and passive? 



ment and inactive as compared with the male cell. It often 

 has an especially well-developed cell-covering. Its nucleus is 

 sometimes called the germinal vesicle and its nucleolus, the 

 germinal spot (Fig. 8). The ovum must be distinguished 



