THE BEARERS OF THE HERITAGE 301 
The scheme of this formation of the polar bodies is indicated in 
Fig. 48. In Fig. 48, 5 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, f, 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 odgenesis. 
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 int» 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 spermatozoén pene- 
trates the wall of the ovum and after undergoing considerable altera- 
tion 1ts 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 spermatozoén unites with an egg. In some 
forms while several may enter the egg only one becomes functional. 
As soon as the nucleus of the spermatozoén, 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 
