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 (o6tid) 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 hes 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 le 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 
