REPRODUCTION _ 187 



Loeb found that if the unfcrtihzed eggs of a sea urchin be placed for 

 about two hours in sea-water whose osmotic pressure has been increased 

 a definite amount by increasing the proportion of salts to water, and then 

 replaced in normal sea water, the eggs segment and develop into swimming 

 larvae. Other rhethods were employed for the initiation of development 

 of eggs of other animals, as starfish, mollusks and annelids. Some of the 

 parthenogenetic agents used are the fatty acids, saponin, solanin, bile 

 salts, the solvents of lipoids (as benzol, toluol, amylene, chloroform, 

 aldehyde, ether, alcohols, etc.), bases, salts, hypertonicity and hypotoni- 

 city of solutions, and rise in temperature. Serum extracts of cells or of 

 sperms of unrelated species or even contact with living foreign sperms 

 as those of the shark, fish or fowl sometimes cause eggs to segment. 

 Shaking the eggs of certain marine animals, stroking the eggs of moths, 

 and pricking the egg of a frog with a fine needle have also been shown to 

 cause development. In most of these instances development does not 

 proceed very far, but the problem of fertilization is in part concerned 

 with the mere initiation of development. 



The immediate cause of development of an artificially parthenogenetic 

 egg is uncertain. In normal fertilization of an egg by a sperm, it has 

 been shown that the rate of oxidation of the egg increases about 400 to 

 600 per cent., and there is much to indicate that the agents inducing arti- 

 ficial parthenogenesis also increase oxidation. Even if the increase of 

 oxidation is not the real cause of development, that cause is almost 

 certainly some chemical or physical phenomenon. It is known in some 

 cases that the agents which artificially cause segmentation of the ovum 

 alter the colloidal state of the protoplasm." The protoplasm becomes 

 stiffer (more viscous), and this change is followed by segmentation. 

 Toward the close of each division the protoplasm becomes more liquid, 

 but more viscouS again at the beginning of the succeeding division. 

 Similar changes occur at the time of normal fertilization by a sperm, 

 suggesting that the artificial agents operate in the same way as does 

 the sperm. 



Nature of Fertilization. — Hertwig's definition of fertilization as the 

 fusion of sperm and egg nuclei is not wholly satisfactory since in artificial 

 parthenogenesis there can be no union of two nuclei; nevertheless, the 

 egg is fertilized, that is, it is stimulated to develop. Moreover, occasion- 

 ally the sperm and egg nuclei do not fuse before the egg segments; 

 indeed, these nuclei may retain their identity for many cell generations 

 in the dividing egg. A comparison of the results of the work on artifi- 

 cial parthenogenesis with normal fertilization shows that one effect of 

 the sperm is the initiation of development and that this initiation is due 

 to the presence and action of substances introduced by the sperm which 

 produce certain chemico-physical effects. The sperm nucleus also brings 

 in, as pointed out in Chapter XI, material bearing hereditary qualities. 



