CHAPTER II 



CLEAVAGE 



*" 



THE fertilized ovum is ready to develop, and the first step 

 in development is segmentation or cleavage. 



The segmentation of the ovum may be said to present to 

 us, on the whole, four problems. The first of these is to 

 discover the reason why the egg divides at all. The second 

 is to find out why, when it does divide, it exhibits its own 

 particular pattern of cleavage. In the third place we have 

 to inquire into the causes which bring segmentation as such 

 to an end. The fourth question asks whether cleavage is or 

 is not in itself a process of differentiation. 



These problems we shall discuss in order. 



1. It is hardly necessary to say that any knowledge we 

 may have of the nature of the causes which provoke the 

 division of the egg is due to the genius of the American 

 physiologist Jacques Loeb, for Loeb has shown that it is 

 possible to replace the stimulus which is normally given to 

 the egg by the spermatozoon, by the action of a solution, that 

 is, by a physical or chemical agency. It is true that others 

 had previously attempted in this way to cause the egg to 

 divide, notably Hertwig and Morgan, but these met with 

 little success. To Loeb belongs the honour of the achieve- 

 ment. 



Since the methods employed have undergone some modi- 

 fication we may briefly recapitulate the history of these 

 experiments. 



The subject was in the first instance the egg of a Sea- 

 urchin (Arbacia, and in later experiments, Stron-gylocen- 

 trotus), and it was found possible to incite it to segment and 

 develop by temporarily immersing it in sea-water to which 



