FACTORS IN ONTOGENY 



285 



X^ 



of fertilization and partlienogcnesis. Jacques Loeb has been 

 the most active worker in this hne and his results are of ex- 

 treme interest. He has, by various physical or chemical treat- 

 ment of the unfertilized eggs of various animals, particularly 

 certain Echinoderms, worms and fishes, stimulated these eggs 

 to begin development, which development proceeds either nor- 

 mally or in some degree abnormally along the usual path reg- 

 ularly followed by the species. But in all cases this develop- 

 ment falls short of completion and in many cases the death of 

 the embryo occurs at a very early stage. Other investigators 

 have similarly induced a de- 

 velopment in parthenogenetic 

 eggs of animal species in 

 which parthenogenetic devel- 

 opment does not occur nat- 

 urally, or at least is very rare. 

 The significance of these 

 results is by no means wholly 

 clear. Nor do the investiga- 

 tors who have done the work 

 agree among themselves as to 

 the interpretation of the re- 

 sults. Loeb first inclined to 

 the belief that the stimuli 

 which incited the unfertilized 

 egg to development were 

 physical, osmotic changes be- 

 ing looked on as perhaps the 

 immediate stimulus. At pres- 

 ent he seems inclined to at- 

 tribute the stimuli rather to 



the chemical character of the media which seem to incite 

 the parthenogenetic development. In either case the physi- 

 cochemical stimulus is considered to be a substitute for the 

 spermatozoid. That it is a substitute in some degree, is obvi- 

 ous; that it is a complete substitute for it, seems e(]ually 

 obviously not true. The embryos develoi")ed by artificial pur- 

 theogenesis lack at least two fundamentally important attri- 

 butes which tlie young of bisexual jiarentage possess; namely, 

 vigor and the hcTcdity of tlu' father. The lack of vigor is 

 shown by their death before maturity; and the chromosomes 



Fig. 168. — Regeneration of the flatworm, 

 Planaria lu(}iibris: A, shows by dotted 

 line where the worm was cut in two length- 

 wise ; B, C, D, show how a half that was 

 fed regenerated ; E, F, G, show how an 

 unfed half regenerated. (After Morgan.) 



