784 DEVELOPMENT 



ance of the nuclear membrane, and become arranged upon a nuclear spindle, 

 as if for an ordinary cell division. The two maturation divisions occur in 

 rapid succession. In the first of these the oocyte is divided extremely un- 

 equally, into a secondary occyte of nearly the same size as the primary, and the 

 first polar body, morphologically equivalent to a minute, degenerant secondary 

 oocyte. Each secondary oocyte receives a single set of diads, which are the 

 division products of the set of tetrads. The second maturation division is 

 similarly unequal, and its products are the ripe ovum and the second polar 

 body. Each of these receives a single set of simple chromosomes. Those 

 that are in the ovum become the female pronucleus, possessing the reduced 

 chromosomal number, figure 500. The centrosome disappears, and the egg 

 is left incapable of division. It remains unchanged unless fertilized by the 

 sperm. 



Material Basis of Heredity. In considering the relations of parent and 

 child to each other through heredity, it is necessary to distinguish sharply 

 between that which comes to the child as an inherent part of its nature, and 

 all else which may be added through conditions of nourishment, physical 

 surroundings, infection with disease-producing parasites, and the like. 

 The former is the only true heredity, in a scientific sense, although it is 

 obvious that until birth, or even for some time after birth, the well being and 

 development of the germ cells, fetus, and child are directly dependent upon 

 the parental organism. 



It has been shown that every cell in a child is descended from one cell, the 

 fertilized ovum. This fertilized egg is the product of the union of male and 

 female germ cells, cells which are not derived from the active body tissues of 

 the parents, but can be traced back to cells that were set aside for the purpose 

 of reproduction at a very early stage of the parent's embryonic life. There- 

 after the influences which the vicissitudes of the parent's life may have exerted 

 upon the germ cells he produces are limited to what can be carried by diffu- 

 sion into the germ cells of substances from the lymph. 



Relative Influence of Mother and Father in Heredity. We may use a 

 more concrete form of this question as follows: If two races are crossed, does 

 it make any difference which form was the mother's stock, and which the 

 father's ? This comparison has been made very extensively in domesticated 

 animals and garden plants, and to a less extent in man. Good examples are 

 the inheritance of color peculiarities in mammals, and peculiarities of the 

 skeleton. Excepting in certain special cases, like color blindness in man, 

 which will be treated later, it is found that these "reciprocal crosses" pro- 

 duce identical results in heredity; hence, that the average influence of father 

 and mother upon inheritance is in general equal. We have already seen 

 that the two sexes make very unequal contributions to the substance of 

 the fertilized egg. The following table summarizes these differences 

 and similarities. 



