CHAPTER XIX. 



PROCESSES INFERRED FROM INDIRECT OBSERVATION: 

 FERTILIZATION AND EARLY DEVELOPMENT. 



THE SUBSTITUTION OF CHEMICAL AGENCIES FOR NORMAL 

 FERTILIZATION. 



IN the vast majority of animal forms the stimulus of fertilization by 

 a spermatozoon of the same or a very closely related species is essential 

 for the development of the egg. The fact, however, that Partheno- 

 genesis, or development without fertilization may occur under excep- 

 tional circumstances or in a limited number of forms, shows that the 

 part played by the spermatozoon, in so far as it constitutes the stimulus 

 to development, may be performed by other agents. The discovery 

 of the exact nature of agents capable of giving rise to development 

 of the egg was essential to the understanding of the phenomenon of 

 Fertilization, for the spermatozoon, besides affording to the egg the 

 initiatory impulse to development also acts as a bearer of hereditary 

 factors and is, moreover, itself a living and a motile organism so that a 

 great complexity of materials and factors gain entry into the egg with 

 the introduction of the spermatozoon, and the disentanglement of 

 these numerous variables was impossible until a clue to their nature 

 had been obtained by means of experiments in which the single func- 

 tion of fertilization was imitated by physicochemical means. 



The solution of this problem we owe to the investigations of J. Loeb 

 who followed up the observation of T. H. Morgan and others that 

 unfertilized eggs of various marine organisms may occasionally begin 

 to segment without fertilization in sea-water, but that such eggs 

 invariably die after a few divisions. In seeking to ascertain the 

 origin of this abnormal phenomenon Loeb found that in the eggs 

 of a sea-urchin, Arbacia, development could be induced by merely 

 exposing them for a period to slightly Hypertonic Sea-water and 

 then returning them to normal sea-water. The means employed 

 to render the sea-water hypertonic was, within certain limits, imma- 

 terial. Thus the Osmotic Pressure might be raised by spontaneous 

 evaporation, or by the addition of one part by volume of 1\ normal 

 sodium chloride solution to nine parts by volume of sea-water, or yet 

 again Cane-sugar or Urea or some other physiologically inert substance 

 might be employed for this purpose and with like success. It was 

 even found possible to cause development of the eggs by immersing 

 them in a pure cane-sugar solution only slightly exceeding sea-water 



