14 THE DEVELOPMENT OF THE CHICK 



and the ovum. The size of the polar globules is usually so small 

 that their elimination makes no appreciable difference in the 

 size of the ovum proper, but they have, nevertheless, the same 

 nuclear constitution as the ovum. 



The mature ovum (ootid) and the polar bodies are the precise 

 equivalent of the four spermatids, but whereas each of the latter 

 becomes a functional spermatozoon, only the ovum on the female 

 side is functional; the polar bodies lack the necessary protoplasm 

 and yolk for development, and they therefore die. The polar 

 bodies must be regarded as abortive ova; and a teleological ex- 

 planation of the form of maturation of the ovum is afforded by 

 the consideration that equal maturation divisions would reduce 

 the amount of protoplasm and yolk in the products below the 

 minimum desirable for perfect development. 



Although the maturation divisions of the ovum and sperma- 

 tozoon are so dissimilar externally, yet the nuclear phenomena 

 are exactly alike. The net result of the maturation divisions is 

 to produce definitive germ-cells containing one half of the somatic 

 number of chromosomes owing to the reduction by pairing (syn- 

 apsis) that occurs in both at the beginning of the period of growth. 

 The somatic number is again restored when the sperm-nucleus 

 and the egg-nucleus unite in fertilization. Questions of funda- 

 mental importance for the problems of heredity arise in connec- 

 tion with the phenomena of maturation and fertilization, but 

 their consideration lies without the scope of the present book. 



VI. POLARITY AND ORGANIZATION OF THE OVUM 

 Although the ovum is morphologically a single cell, yet, as 

 the primordium of an individual, it has certain specific properties 

 that predelineate or foreshadow the main structural features of 

 the embryo. Polarity is the most general of these features: all 

 the axes of the ovum are not similar, though they may be equal; 

 there is one axis around which the development centers ; the ends 

 of this axis are known as the animal and the vegetative poles of 

 the ovum, and the hemispheres in which they lie are named 

 correspondingly. In telolecithal ova the yolk is centered in the 

 vegetative hemisphere, the protoplasm in the animal hemisphere; 

 even in ova which are called isolecithal there is a tendency for 

 the yolk to be more abundant in the vegetative hemisphere. 

 The polar globules are formed at the animal pole; hence their 



