3 o4 GENERAL BIOLOGY 



plants at least. At the beginning of the growth period that 

 precedes the two maturation divisions there occurs a fusion 

 of the chromosomes within the nucleus in pairs, apparently 

 with like paternal and maternal elements in each pair (these 

 elements having, since the preceding fertilization, main- 

 tained themselves apart, although in one nucleus). This 

 fusion is called synapsis. It brings into more intimate asso- 

 ciation the equivalent paternal and maternal units, appar- 

 ently commingling their substance, and possibly merging 

 the influences they have borne separately since the preced- 

 ing fertilization brought them together. 



Parthenogenesis. New individuals develop from eggs, 

 and not from sperms alone, but either eggs or sperms alone 

 seem to have the necessary nuclear equipment for the com- 

 plete development of new individuals; the eggs alone, have 

 the cytoplasmic equipment necessary. In many large and 

 widely separated groups of animals, (aphids and other in- 

 sects, daphnids and other crustaceans, rotifers, etc.), there 

 occurs habitually the development of eggs without fertil- 

 ization. This is called parthenogenesis (parthenos, virgin and 

 genesis) . In the honey bee, all the drones are believed to be 

 developed from unfertilized eggs. This phenomenon is 

 usually an accompaniment of peculiar conditions of life, and 

 it alternates at longer or shorter intervals with true sexual 

 reproduction. 



In the vast majority of organisms, however, the addition 

 of the sperm to the egg is a necessary stimulus that must be 

 supplied before development proceeds. But the eggs of a 

 number of animals that ordinarily require fertilization can 

 be artificially stimulated to develop by temporary immer- 

 sion in proper alkaline solutions. 



The sperm nucleus also totipotent. If an egg be deprived 

 of its nucleus, the cytoplasm dies ; it has no power to develop 

 alone. But enucleated eggs have been fertilized and caused 



