18 INTRODUCTORY I 



occurrence of certain natural monsters (Hemitheria anteriora, 

 for example) in which one half of the body is normally de- 

 veloped, the other entirely suppressed, and the experimental 

 demonstration of the formation of a half-embryo from one of 

 the first two blastomeres of the Frog's egg when its fellow had 

 been killed, all led Roux to regard the development of the whole 

 and of each part as essentially a process of self-differentiation, 

 a process, that is to say, of which the causes reside wholly within 

 the fertilized ovum and within each part as it is formed, though 

 allowance was made for the possible formative influence of the 

 parts on one another in later stages. External conditions, though 

 they may be necessary in the same sense as they are generally 

 necessary to the maintenance of life, are yet of no importance 

 for differentiation regarded as a specific activity of the organism. 



In the meantime, an experiment of Pfliiger's had apparently 

 shown that, however obviously each part of the egg-cell might 

 be related to the production of a particular organ, the relation 

 was no necessary one, but that, on the contrary, the parts were 

 all equivalent and the ovum 'isotropic'. Pfliiger demonstrated 

 that in a Frog's egg which had been prevented from assuming 

 its normal position with the axis vertical, the planes of the 

 segmentation furrows bore no constant relation to the original 

 egg-axis, that is to say to the structure of the egg, though 

 they exhibited the same relation to the vertical as when de- 

 veloping in the normal position. Further, in such forcibly 

 upturned eggs the plane which included the original egg-axis 

 and the present vertical axis became the median plane of the 

 embryo, whose axes were disposed with regard to the vertical as 

 in normal cases. Any part of the egg, therefore, might give 

 rise to any part of the embryo, according to the extent to which 

 and the direction in which the egg-axis had been diverted out 

 of its original vertical position, and hence the egg-substance was 

 ' isotropic ' ; the planes of segmentation and the embryonic axes 

 being determined by gravity. In fact, Pfliiger went so far as 

 to say that an egg only becomes what it does become because it 

 is always placed under the same external conditions. 



Nor was this conception of the isotropy of the ovum invali- 

 dated by Bern's proof that in these eggs there is a redistribution 



