Io8o REPRODUCTION 



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. 



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 

 of a normal solution, or about 14-6 per cent.) has been added, those 

 of the eggs which have formed membranes develop into swimming 

 larvas that rise to the surface. These larvas develop into perfect 

 sea-urchin larvas or ' plutei ' as fast as the larvas of eggs fertilized 

 with sperm. 



The facts of parthenogenesis show that it is not absolutely neces- 

 sary for development that the ovum should have the normal number 

 of chromosomes restored. It can develop with half the number, the 

 chromosomes of the female pronucleus being sufficient for growth, 

 although, of course, in this case for a growth uninfluenced by the 

 properties of the male element. In like manner it is stated that 

 portions of the maturated ovum devoid of a nucleus can undergo 

 development if penetrated by a spermatozoon, the chromosomes of 

 the male pronucleus being sufficient for growth. 



