SYNGAMY 247 



tion disjunction occurs in the third or gamete-producing mitosis, this 

 mitosis being the one which is lacking in endomixis. Molecular rear- 

 rangements doubtless occur, but to what extent these may affect the 

 constitution or action of the physical basis of heredity remains for 

 future work to determine. 



The Physiology of Fertilization. — The problem of identifying the 

 physico-chemical changes involved in the activation of the egg has been 

 attacked through studies of artificial parthenogenesis, as well as through 

 direct analyses of the chemical constitution of the gametes at these 

 stages. ^^ 



In 1899 Jacques Loeb made the important discovery that the parthe- 

 nogenetic development of certain animal eggs can be artificially induced. 

 In most cases two successive treatments were found necessary; first, a 

 brief treatment with sea water containing some permeability-increasing 

 substance (certain fatty acids, bases, soaps, alkaloids, glucosides, and 

 foreign blood sera) ; and, second, a longer treatment with hypertonic sea 

 water, or oxygen-free sea water, or sea water with a trace of KCN, or low 

 temperature. The first treatment caused the formation of a fertilization 

 membrane, which seemed to be a condition necessary for continued 

 development. The critical change was apparently a momentary increase 

 in surface permeability which set in motion the developmental reactions, 

 but in most cases the eggs became sickly and died unless given the second 

 treatment. Loeb concluded that the spermatozoon in normal fertiliza- 

 tion may carry two substances, one of which produces the necessary 

 initial surface changes, even when the spermatozoon fails to enter the 

 egg, while the other in some way acts as does the second treatment in 

 artificial parthenogenesis. This suggestion seems less plausible in the 

 light of Just's (1922) discovery, that normal sea-urchin larvse can be 

 produced by treating the eggs with hypertonic solutions alone. 



Loeb thought the sickliness of eggs given only his first treatment to 

 be due to a continuance of the cytolytic action begun by this first treat- 

 ment, but F. R. Lillie pointed out the greater probability of the view that 

 it is due to some internal cause, citing as supporting evidence certain 

 cytological phenomena observed by Herlant in eggs activated by Loeb's 

 method. During the healthy period (12 to 24 hours) immediately 

 following the first treatment, Herlant (1917) observed the following 

 changes. After the formation of the membrane and a hyaline zone, 

 the nucleus becomes the seat of a series of conspicuous alterations. The 

 nuclear membrane disappears, and around the chromosomes there is 

 formed a one-poled achromatic figure but no amphiaster. The chromo- 



38 For general accounts, see Loeb (1913), F. R. Lillie (1919), Lillie and Just (1924), 

 R. S. Lillie (1923), and Gray (1931). For work on plants, see J. B. Overton (1913) 

 on Fuciis, Tahara (1927) on Sargassum, and Popoff (1920, 1922, 1931) on somatic 

 tissues. 



