DEVELOPMENT OF THE OVUM 1123 



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 spermatozoon 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 centriiugalization 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. 



Parthenogenesis. Attempts have been made to separate the 

 constituents of spermatozoa which are essential to fertilization. 

 From the sperm of a sea-urchin a substance can be extracted by 

 strongly hypotonic salt solutions, containing ether, which acts as a 

 powerful fertilizing, agglutinating, and cytolyzing agent upon the 

 eggs. It is soluble in dilute acid, and is probably identical with a 

 fertilizing agent called oocytase present in blood-serum (Robertson). 

 Whatever it is that the spermatozoon supplies, the process of 

 fertilization can in certain forms be started artificially in the absence 

 of spermatozoa or any of their constituents. 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 

 spermatozoon. When these eggs are afterwards exposed for thirty 

 to forty minutes to 100 c.c. of sea-water, to which 14 or 15 c.c. of a 

 strong solution of sodium chloride (two and a half times the strength 



