INTERNAL CAUSES OF VARIATION 167 



remaining in the cell, the other passing outside, forming the 

 first polar body, which may or may not undergo further division. 



The portion now remaining within the cell consists of groups 

 of two each, instead of four, and their number is of course the 

 same as before, namely, one half the somatic number of chromo- 

 somes. Immediately now, without assuming the resting stage, 

 the dyads, or groups of twos, turn one fourth around, taking a 

 position at right angles to the margin of the cell, and at once 

 divide again, one member of each pair remaining behind in the 

 egg cell, the other passing out, forming the second polar body. 



The first polar body carried away one half the nuclear matter, 

 and the remaining half has now been divided equally between 

 the second polar body and the main cell, which is now ready for 

 fertilization and is from this time on spoken of as the ovum. 



Neither polar body carries any appreciable quantity of cyto- 

 plasm, and both are destined to degenerate and disappear. The 

 first one, however, containing half of the total nuclear matter, 

 commonly divides once, so that the first polar body represents 

 not one, but two cells, the first and the third polar bodies. 



The total result, then, of this complicated process seems to 

 be the equal division of the chromatin matter between the 

 ovum, capable of fertilization, and three polar bodies, destined 

 to extinction. 



A group of four cells thus arises, - namely, the mature egg 

 (ovum), which after fertilization gives rise to the embryo, and 

 three small cells or polar bodies (incapable of fertilization), 1 

 which take no part in the further development, are discarded, 

 and soon die without further change. The egg nucleus (of the 

 ovum proper) is now ready for union with the sperm nucleus, 2 

 which process is known as fertilization. 



" In some cases for example in the sea urchin the polar 

 bodies are formed before fertilization, while the egg is still in 



tetrad form is always chosen for description because the details are capable of 

 more definite statement. Whatever the form of the masses, however, the final 

 result seems always the same ; namely, a reduction to one half the usual number 

 of chromosomes, and this by the method of division and extrusion. 



1 In rare instances the polar bodies have commenced to segment, but they 

 never proceed far in development. 



2 Wilson, The Cell, pp. 236-237. 



