THE BEARERS OF THE HERITAGE 301 



The scheme of this formation of the polar bodies is indicated in 

 Fig. 48. In Fig. 4^, b the chromosomes are seen paired and ready for 

 the first division; that is, for the formation of the first polar body. 

 Figs. 48, c, d show the giving off of this body. Note that while only a 

 small proportion of the cytoplasm passes into this tiny cell, its chro- 

 matin content is as great as that of the ovum. A second polar body 

 (Fig. 48, e) is formed by the egg, but in this case each chromosome 

 splits lengthwise, as in ordinary mitosis, and there is no further numeri- 

 cal reduction. In the meantime, typically, a third polar body is 

 formed by division of the first. (Stages e, /, g.) 



Parallel between the maturation of sperm- and egg-cell. — This 

 rather complex procedure of the germ-cells will be rendered more 

 intelligible through a careful study of Figs. 47, 48, and 49, which 

 indicates the parallel conditions in spermatogenesis and oogenesis. 



The view now generally held regarding the polar bodies is that they 

 are really abortive eggs. They later disappear, taking no part in 

 embryo-formation. It can readily be seen how such an unequal 

 division is advantageous to the large cell, for it receives all of the rich 

 store of food material that would be distributed among the four cells 

 if all were of equal size. This increased amount of food is a favorable 

 provision for the forthcoming offspring whose nourishment is thus 

 more thoroughly insured. 



On the other hand, all of the sperm-cells develop into complete 

 active forms, which, as aforesaid, usually become very much elongated 

 and develop a motile organ of some kind. In such cells an accumula- 

 tion of food to any large extent would hinder rather than help them, 

 because it would seriously interfere with their activity. 



Fertilization.— In fertilization (Fig. 50) the spermatozoon pene- 

 trates the wall of the ovum and after undergoing considerable altera- 

 tion Its nucleus fuses with the nucleus of the egg. In some forms 

 only the head (nucleus) and middle-piece enter, the tail being cut off 

 by a so-called fertilization membrane which forms at the surface of 

 the egg and effectually blocks the entrance of other spermatozoa. 

 Thus normally only one spermatozoon unites with an egg. In some 

 forms while several may enter the egg only one becomes functional. 

 As soon as the nucleus of the spermatozoon, now known as the male 

 pronucleus, reaches the interior of the egg, it enlarges and becomes simi- 

 lar in appearance to the female pronucleus. It swings around in such 

 a way (Fig. 50, b) that the middle piece, now transformed into a centro- 

 some, lies between it and the female pronucleus. The two pronuclei 



