244 Egbert Thompson Young, 



may persist thru several cell-generations is possible; but my obser- 

 vations indicate that, in Monie^ia at least, it is not of fundamental 

 significance, being merely the product of temporary conditions and 

 persisting as long as these persist. . . . The cells which give rise 

 to the sexual cells in Moniesia do not differ visibly in any way 

 from the cells of the parenchyma. Apparently those nuclei which 

 are affected by certain conditions develop into sexual cells while 

 those not thus affected may develop in other directions. Yet the 

 parenchymal syncytium . . . shows a considerable degree of cyto- 

 plasmic differentiation ... its cells are essentially tissue cells, not 

 a reserve stock taking no part in the formation of the body. Never- 

 theless some of them become germ cells ... I believe that these 

 facts possess a certain significance as indicating that there is no 

 fundamental and continuous distinction between tissue cell and germ 

 cell. The cells become what conditions determine. In my opinion 

 the cell, so far as it is an individual at all, is primarily a physio- 

 logical and not a morphological individual." 



With most of Child's conclusions I heartily agree. In view, 

 however, of the extremely degenerate character of the Cestodes, great 

 care should be exercised in applying to other forms any conclusions 

 deduced from observations made in this group. That the cell "is 

 primarily a physiological and not a morphological individual" however, 

 may be true in forms other than the Cestodes. The well known 

 ability of certain animals to regenerate lost parts from parts un- 

 related to them is of significance in this connection. 



The difference in staining reaction in different cells due, in ni}'' 

 opinion, solely to the amount of nuclear material stored up in them 

 at any time and not to any constant, specific morphological differ- 

 ence, has been mentioned by various authors, notably Lang (1881), 

 HoFMANN (1899) and Rösslee (1902). 



At the end of each section, has already been given in detail 

 a summary of the facts of histogenesis and cytogenesis in Cysti- 

 cercus pisiformis. It remains only to emphasize the most im- 

 portant points in the following: 



5. Conclusions. 



Cysticercus pisiformis presents to us an extremely simple 

 type of histogenesis, the various tissues being developed exclusively 

 by modification in situ of a pre-existent undifferentiated tissue — the 



