SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 431 



the activities which these substances are called upon to excite or 

 direct the most complex are those which have to do with their 

 own anabolism. On this anabojism depends the reproduction of 

 like individuals through sex cells. 



The belief that substances which govern differential morpho- 

 genesis reside chiefly in the nucleus is based, in the main, on the 

 fact that the spermatozoon plays a part equal to that played by 

 the egg in transmitting inheritable characteristics. Since the 

 chief part of the spermatozoon is the chromatic material in the 

 head and since this chromatic material gives rise directly to half 

 of the nuclear material of the fertilized egg the belief seems well 

 grounded. Those who, like Meves, contend that there are spe- 

 cific substances in the cytoplasm which play an equally or nearly 

 equally important part in the transmission of heritable characters, 

 have as yet offered no convincing proofs. That certain cyto- 

 plasmic substances or structures, centrosomes, chondriosomes, 

 and the like, may be passed on from parent to daughter cells 

 through spermatozoa as well as through ova does not in any way 

 prove that these structures are important determinants of the 

 subsequent morphogenic differentiation. Until further proof is 

 brought forward in support of the importance of the influence of 

 the cytoplasm in determining differential morphogenic activity 

 it seems safe to follow Hertwig in assuming that the chief deter- 

 minants lie in the nuclei. The nuclei reside in and work on the 

 cytoplasm, so that the power of the nucleus to determine differ- 

 ential morphogenesis is determined in turn to some extent by the 

 cytoplasm, much as the activity of any organism is determined 

 by its environment. 



It is well known that the most characteristic chemical sub- 

 stances in the nucleus are the nucleins, complex organic com- 

 pounds containing phosphorus. It seems quite probable that 

 the more complex of these compounds are the cell elements most 

 easily disturbed by the X rays. The X rays are known to 

 have the power of ionizing certain chemical substances. It seems 

 quite probable that they may break down the more complex 

 nuclear substances in such a way that the latter cannot be per- 

 fectly restored. The loss of these substances may be noted 



