REPRODUCTION 1007 



by the fibres of the spindle. It results, f rom this that two daughter 

 nuclei are formed, each with the same number of chromosomes 

 as the original nucleus, although with only half the amount of 

 chromatin. The cytoplasm divides also, 'so that the parent cell is 

 now represented by two daughter cells. In ordinary cell division 

 the two daughter cells are of equal size, but in the division of the 

 ovum which occurs before fertilization the two resulting cells are 

 very unequal. The large cell continues to be known as the ovum ; 

 the small one is the first polar body. After extrusion of the first 

 polar body the ovum again divides unequally. A new spindle forms, 

 and a second polar body, again much the smaller of the two daughter 

 cells, is cast off. There is a difference, however, between the process 

 of division which gives rise to the first and that which gives rise 

 to the second polar body. In the case of the latter a so-called 

 reduction-division occurs ; the chromosomes do not split longi- 

 tudinally, but half of the original number pass into each daughter 

 nucleus. As to the significance of these changes there has been 

 much discussion. It is agreed that the result of the process is the 

 expulsion of a portion of the chromatin, the ovum now possessing 

 only half the original number of chromosomes, although nearly all 

 the original cytoplasm. In fertilization the original number is 

 restored by the male element when it arrives and penetrates the 

 ovum. For in the final cell-division by which the mature sperma- 

 tozoon is formed the chromosomes of its nucleus are also, after two 

 divisions essentially similar to those occurring in maturation of the 

 ovum, reduced to half the normal number. 



The two reduced nuclei in the fertilized ovum are spoken of as 

 the male and female pronuclei. By their union a single nucleus is 

 formed with the number of chromosomes normal to the species. 



An enormous amount of interesting work has been done with the 

 view of illustrating the connection of the complicated phenomena 

 described with the structure of the ovum. Only a bare reference 

 to one or two of the experiments is possible here. Driesch and 

 Hertwig find that the nucleus can be made artificially to change its 

 place with reference to the yolk, without hindering the development 

 of a normal animal. Lillie has shown that centrifugalization of the 

 eggs of annelids, although it markedly alters the distribution of the 

 yolk and other substances, does not affect the form of cleavage. 

 The polar bodies appear in the position which they would normally 

 occupy. In other words, no redistribution of the granules or nucleus 

 affects the polarity of the egg, which therefore is a function or 

 property of the ground substance of the protoplasm. The whole 

 of the protoplasm, however, is not necessary for complete develop- 

 ment. Even in Amphioxus, the lowest of the vertebrates, the 

 eggs have been broken up by shaking, and a complete animal 

 evolved from as little as one-eighth of an ovum. If the separation 

 was incomplete a kind of Siamese twins, or even triplets, could be 

 obtained (Wilson and Mathews). Nor is it always indispensable 

 that both pronuclei should be present. 



Whatever it is that the spermatozoon supplies, the process of 

 fertilization can in certain forms be started artificially. The studies 

 of Loeb and his pupils on artificially induced parthenogenesis are of 

 special importance. When the unfertilized eggs of the sea-urchin are 

 exposed for one or two minutes to 50 c.c. of sea-water, to which 

 3 or 4 c.c. of decinormal acetic acid has been added, the majority of 

 the eggs form the membrane characteristic of the entrance of the 



