3° 



ZOOLOGY 



46. The Fertilized Ovum as a Starting Point. — In speaking 

 of the development of the adult animal from the simpler con- 

 dition of the single cell it is necessary to remember that this 

 cell, which has the power of giving rise to a complex individual 

 and is called a fertilized ovum, has a history of its own 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. 



47. The Ovum. — The germ cell produced by the female is 

 known as the ovum, and is typically a spherical cell with 



Fig. 9. Types of ova. A, pripiitive amceboid ovum of Sponge; B, semi-diagrammatic figure 

 of spherical ovum of Sea-urchi7i 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 yolk 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? 



abundant nourishment 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 

 (Fig. 9). The ovum must be distinguished from what is 

 popularly known as an egg. The latter term is loosely used 

 to describe the fertilized ovum more or less developed, to- 

 gether with its nutritive and protective coats such as occur 



