Sfj-t Tlie Unit 2/ of the Organism 



ole and excretory pore." The question which chiefly con- 

 cerns us now, but which received no consideration in the 

 earlier treatment is, what part, if any, does the chromatin 

 of the nucleus play in the initiation and development of these 

 organs? One of two courses must be followed if the 

 chromatin theory is to be proved in a specific instance like 

 this : either the developmental facts presented must be 

 shown not to be subject to heredity or it must be proved 

 that they are caused by the chromatin. 



That many modern students of heredity have strongly 

 tended by implication if not expressly to pursue tlie first 

 mentioned course, cannot be* successfully disputed. This 

 was dwelt upon in the early part of our discussion of hered- 

 ity and we may hope its utter unwarrantableness was re- 

 vealed. As a consequence our only task now is to inquire 

 what the evidence is that the developments before us are 

 causally explained by the nuclear chromatin — or for that 

 matter by chromatin of any other kind. 



The method of handling the evidence, not only in this 

 particular case, but in all others with which we shall deal, 

 must be stated at the outset. Briefly, our task is not to 

 prove what chromatin does not do, but to point out what 

 cytoplasm and other substances do in connection with the 

 development of tlie organs under consideration. Otherwise 

 stated, just as in the effort to decide whether or not chro- 

 mosomes and chromatin are the physical basis of heredity, 

 we sought for evidence of the direct participation of these 

 in the production of organs and parts, so now we have to 

 inquire as to whether or not extra-nuclear and non-chro- 

 matic parts of the cell participate in the production of 

 organs and parts. 



"The first sign of fission," Johnson has already been 

 quoted as saying, "is the formation of a rift (the anlage 

 of the new aboral zone) in the pelHcula and ectoplasm, near 

 to and almost parallel with the left boundary stripe of the 



