174 MARINE BIOLOGICAL LABORATORY. 



It is now quite generally conceded that the nucleus 

 of the fertilized ovum contains all the hereditary charac- 

 teristics of the parent organisms. It is this substance 

 in the ovum which stamps the particular characteristics 

 upon an organism of a given species. The study of 

 fertilization has clearly demonstrated the metamorphosis 

 of the sperm-nucleus into a constituent part of the 

 cleavage-nucleus, and thence it is distributed to all 

 nuclei formed in the subsequent cleavages. Morpholog- 

 ically, all the hereditary characteristics which the infant 

 organisms inherit from the parents, must be traced back 

 to a certain number of chromosomes which come from 

 the sperm and egg-nuclei of the fertilized ovum. By 

 cleavage, the potential characteristics become gradually 

 analyzed into their special attributes — the attributes 

 which we assio:n to different tissues of the larval or the 

 adult organism. If, therefore, I may use one word to 

 characterize the whole process of cleavage of the ovum, 

 the term Analysis will perhaps best express our interpre- 

 tation of the phenomenon. It is true, that we know 

 very little as to the essential respects in which the 

 nuclear substance in the entodermic cleavage sphere 

 differs from the similar substance in the ectodermic 

 sphere. In the present state of our knowledge on this 

 subject, we can only infer a structural difference of the 

 protoplasm from the careful study of the fate of the 

 respective segments. If, for instance, one cell gives 

 rise to a sense-organ, the fundamental molecular struc- 

 ture of that cell must be different from another which 

 contains all the germs of an excretory organ, just as we 

 are forced to conclude that the ova of different organ- 

 isms are of necessity different, even if they appear 



