REPRODUCTIVE CELLS 17 



whole process represents a twice-repeated cell-division, resulting in one 

 large cell (the matured ovum) and three small polar cells (not often com- 

 pletely divided), that, on account of their small size, appear to arise by a 

 process of budding from the egg; also that each of the four cells contains 

 only half the normal number of chromosomes for somatic cells of the same 

 animal, and, since the polar cells take no further part in development, the 

 matured egg has come into possession of a greater amount of proto- 

 plasm, with its store of yolk, but only half the number of chromosomes, 

 which form into a new nucleus of smaller size — the female pronucleus 



(fig. 9). 



Coincidentally with these events, belonging properly to the egg, 

 there are also processes, partly parallel with them, taking place in the 

 transported head of the sperm-cell. In spermogenesis, as well as in 

 oogenesis, there are three periods, proliferation, growth, and maturation. 

 The first two in the case of the egg, but all three in the case of the sperm, 

 take place in the reproductive organ of the parent, so that maturation 

 divisions in the latter case only contribute to increasing the number of 

 spermatozoa, since there are no sterile, degenerate polar bodies among 

 them, those cells corresponding to polar bodies maturing into sperms and 

 quadrupling the number of the latter. The tail of the impregnating sperm 

 either drops off at the surface of the egg or becomes absorbed into the egg- 

 protoplasm. The head becomes changed into a nucleus — the male pro- 

 nucleus (fig. 9) — and it is believed the neck or point of insertion of the tail 

 carries over a centrosome, which becomes active in the formation of an 

 astrosphere. Male and female pronuclei become attracted to one another 

 along a new spindle, meet and fuse about the middle of the egg, and out of 

 them a new nucleus is constructed — the segmentation-nucleus (fig. 10) — 

 of normal size and number of chromosomes, of which half are descended 

 from the ovarian ovum and half contributed by the sperm. The process 

 of fertilization is now complete, the product being a fertilized egg (oosperm, 

 oosphere), which, as shown by its subsequent history, is a very different 

 thing from an unfertilized egg, that will not develop. 



In contemplating the foregoing process it is impossible to disregard 

 the manner in which all parts of the egg seem to be under control, the 

 mechanical way in which some bodies are moved about, and the physico- 

 chemical methods by which others are dissolved and reappear. Another 

 point of interest is the reduction of egg and sperm chromosomes to half 

 the normal number for the species, thus apparently necessitating the union 

 of elements from two different parents, with all their hereditary potentia- 

 lities. Some zoologists may see in this an internal means of bringing 

 about variations among individuals, leaving to natural selection the task 

 of preserving those variations that constitute the best adaptations to 

 external conditions. 



