THEORIES OF DEVELOPMENT 243 



the results of studies of regenerative phenomena of organisms show 

 that the conclusions are not necessarily the only ones deducible from 

 the experiments; for, although it may be true that any possible primary 

 meridian of the egg may become the median plane of the body of the 

 embryo, it does not follow that there is no one organized plane always 

 present in the normal egg, i.e. the egg may not be entirely isotropic. 

 That this may be the case is shown in the regeneration of pieces 

 of adult animals in which a piece cut to one side of the old median 

 plane may develop a new plane of symmetry of its own. This possi- 

 bility must be also admitted for the egg. If we substitute the term 

 " totipotence," meaning that any meridian of the egg has the possi- 

 bility of becoming the median plane of the embryo, in place of 

 Pfliiger's term " isotropy," we remove this element of possible error 

 from his statement. 



Roux and Born have shown that the only action that gravity has 

 on the frog's egg is to bring about a rearrangement of the contents 

 of the egg, a phenomenon that Pfliiger had not observed. The lighter 

 part flows to the highest region of the egg, and the heaviest to the 

 bottom of the egg, hence the change in the position of the cleavage 

 planes observed by Pfliiger that begin in the upper, more protoplasmic 

 part of the egg. 



Another series of experiments, that we also owe, in the first place, 

 to Pfliiger ('84), consist in compressing the egg before and during 

 its cleavage. The position of several of the cleavage planes may be 

 altered, yet a normal embryo develops from the egg. The same 

 experiment has been repeated by Hertwig ('93), and by Born ('93), 

 on the frog's egg, and by Driesch ('92), Ziegler ('94), myself ('93), 

 and others, on the egg of the sea-urchin, with substantially the same 

 results. The value of the experiment lies not so much in showing 

 that the coincidence between the first cleavage planes and the orient- 

 ing planes of the body may be lost, as in showing that under these 

 circumstances the nuclei have a different distribution in the proto- 

 plasm from that which they hold in the normal egg. Any theory of 

 development that depends on the qualitative distribution of nuclear 

 products during the cleavage period meets with great difficulties in 

 the light of these results, and in order to overcome them will be 

 obliged to add qualifications of such a kind as materially to alter 

 its simplicity. Roux's theory, for instance, comes into this category. 

 Roux ('83) suggested that since the complicated karyokinetic division 

 of the nucleus is carried out in such a way as to insure a precise divi- 

 sion of the chromatin, and since the qualities of the male are transmitted 

 to the egg through the chromatin of the spermatozoon, it is probable 

 that the division of the chromatin is a qualitative process, by means 

 of which the elements are distributed to different parts of the egg. 



