204 GENERAL EMBRYOLOGY 



spermathecse, and other similar conditions, clearly indicate that 

 this is a secondary or derived condition, a special adaptation, 

 which therefore throws but little light upon the fundamental 

 significance of the fertilization process; this proves only that 

 fertilization is not in all cases a necessary antecedent to develop- 

 ment, and that the ova may, in certain cases, contain in them- 

 selves complete developmental mechanisms, and need neither 

 the addition of sperm structures nor the special form of stimu- 

 lation afforded by the entrance of the sperm cell. 



Instances of merogony, or "male parthenogenesis, " where 

 egg fragments containing no nuclear substance develop after 

 penetration by a spermatozoon (Echinoderms), also show 

 that the addition of egg and sperm structures, each incom- 

 plete in itself, is not a necessary feature of fertilization. 



It may truly be said that the obviously secondary character 

 of normal parthenogenesis renders the phenomenon of little 

 value as evidence regarding the real meaning of fertiliza- 

 tion in the vast majority of instances. But such an objec- 

 tion cannot be brought against the evidence from experimental 

 parthenogenesis, and probably the clearest evidence upon this 

 phase of the subject is that of " artificial" or induced partheno- 

 genesis. In view of this fact, and of the great general impor- 

 tance of the subject, we may consider this matter rather fully. 



The eggs of many animals, belonging to many different 

 classes and phyla, normally requiring to be fertilized, may be 

 stimulated to begin their development by chemical or physical 

 treatment. Thus the ova of many Coelenterates, Echinoderms, 

 Annulates, Molluscs, and even some Chordata (Teleosts, 

 Amphibia), may be induced to commence their development 

 parthenogenetically by being subjected to the action of a 

 great variety of organic and inorganic substances in solution, 

 to unusually high or low temperatures, to physical shock, or to 

 various other conditions. 



In this process of artificial parthenogenesis two phases 

 must be kept separate, first, the phenomenon of maturation, in 

 those cases where this is not completed at the time fertilization 

 normally occurs; and second, the phenomena of cleavage and 



