124 EXTERNAL FACTORS III. 7 



and others). The increased osmotic pressure was, therefore, 

 considered to be the cause of the phenomenon, and it was 

 suggested that in ordinary fertilization the spermatozoon intro- 

 duces a substance which has a higher osmotic pressure than, 

 and is therefore able to withdraw water from, the egg. 



Hunter has also shown that sea- water concentrated to 70 % 

 of its volume is sufficient to bring about the result. It must 

 still be remembered that the permeabilities of the ova to the 

 various solutions are not known ; Sollmann, indeed, has proved 

 the secondary swelling after the primary shrinkage of many 

 eggs in hypertonic solutions, which must therefore enter and 

 cause the dissociation of the cytoplasm. 



Further, Delage has, as a matter of fact, denied that the 

 increased osmotic pressure is solely responsible for the results. 

 The French zoologist succeeded in making the ova develop in 

 solutions hypertonic to sea-water, but found that isotonic 

 solutions of different chlorides or mixtures of chlorides did not 

 all give the same percentage of larvae. He holds, therefore, 

 that other factors are involved. Other methods, as noticed 

 elsewhere, are low temperatures and mechanical agitation. 



Fischer has successfully demonstrated the phenomenon in the 

 Chaetopods, Nereis and Amphitrite, Bullot in Ophelia, and 

 Bataillon in Vertebrates (Rana fusca and Petromyzon planeri); 

 but in this last case segmentation did not continue for very long 

 and the processes of nuclear division were highly irregular. An 

 attempt made by Gies to incite development (of Echinoids) by 

 means of extracts of spermatozoa was unsuccessful. 



Although in brilliancy of conception and completeness of 

 execution Loeb's experiments are certainly pre-eminent over 

 those of any other investigator, it should not be forgotten that 

 about the same time Morgan had succeeded in inducing asters, 

 and even the beginnings of segmentation, in the unfertilized ova 

 of sea-urchins and some other forms by the use of salts and 

 other substances, and that the way for all recent work was 

 really paved by the original labours of O. and R. Hertwig, to be 

 described in the next section. 



Loeb did not undertake an examination of the cytological 

 changes, but Wilson has shown that ordinary nuclear division 

 occurs with asters and centrosomes : a primary radiation 



