190 THE SCIENCE OF BIOLOGY 



between generations, and on the other were those related 

 to the physiological problem of how the spermatozoon served 

 as an activating agent which stimulated the egg and thus 

 caused its development. The term fertilization has latterly 

 been restricted to the second set of phenomena the problem 

 of how egg and spermatozoon produce a cell capable of 

 division. 



The analysis of fertilization from its morphological stand- 

 point, i. e., the structural features involved in the union of 

 egg and sperm, prepared the way for the physiological 

 analysis now in progress. 2 This analysis consists of (1) 

 work upon artificial parthenogenesis, and (2) biological 

 studies upon what may be termed the fertilization reaction 

 between egg and sperm. The present status of the fertiliza- 

 tion question, as a physiological rather than a morphological 

 problem, illustrates the drift toward experimentation, which 

 has followed upon the establishment of morphological facts, 

 whether in embryo or adult. 



But fertilization does not occur in the development of all 

 eggs, although it is necessarily the starting point in bi- 

 parental reproduction. The phenomenon of natural partheno- 

 genesis, by which the ovum or unfertilized egg-cell develops 

 without the entrance of a spermatozoon, occurs in a con- 

 siderable number of species, among the Insecta, Crustacea, 

 Trematoda, Rotifera, Arachnida, and perhaps the Verte- 

 brata. 3 Males are known to exist in most of these cases and 

 fertilization of the eggs occurs in certain generations, as in 

 the plant lice, or certain eggs are fertilized while others are 

 not, as in the case of the honey-bee. In some cases the males 

 are unknown, but it is presumed that they have not yet been 

 discovered, not that they are absent. Hence, in partheno- 

 genetic species it appears that certain eggs develop without 

 fertilization by the spermatozoa, while other eggs are capable 



2 Lillie, F. R., "Problems of Fertilization," 1919. 



3 Phillips, E. F., "A Review of Parthenogenesis," Proc. Am. Philos. Soc, 

 Vol. XLII, No. 174, 1903. 



