CELL CONSTANCY IN THE GENUS EORHYNCHUS 283 



lin's results on Crepidula exemplify this condition. The number 

 of cells in any given organ may consequently vary within unde- 

 termined limits; (3) Body size is fixed while the size of the 

 component cells may vary. There could be no precise limit to the 

 number of cells in this instance; (4) Body size and cell size may 

 both vary. This last possibility is still further divisible into 

 two conditions : (a) if cell size and body size vary independently 

 no necessary connection exists between the size of the body and 

 the number of component cells, .while (b) if body size varies di- 

 rectly as the size of the cell the number of cells in any given or- 

 gan becomes a fixed quantity, as in the case outlined under (1). 

 It is in accordance with the last possibility that the explanation 

 of the condition found in the genus Eorhynchus is to be sought. 

 In general it is in organisms having this fixed correlation between 

 body size and cell size that cell constancy is to be found. 



Regarding the cell lineage of Nereis, Wilson ('92, p. 377) writes: 

 "The entire ontogeny gives the impression of a strictly ordered and 

 predetermined series of events, in which every cell division plays 

 a definite role and has a fixed relation to all that precedes and 

 fdllows it." How readily this conception of ontogeny lends a 

 support to the theory of cell constancy. In another connection, 

 Wilson, ('00, p. 390) has stated that the number of cells produced 

 for the foundation of certain structures is often fixed. For ex- 

 ample, in annelids and gasteropods ''the entire ectoblast arises 

 from twelve micromeres segmented off in three successive 

 quartets of micromeres from the blastomeres of the four-cell 

 stage." 



By way of explanation of this condition as indicated by 

 Wilson and by Morgan, two possibilities present themselves, 

 either (1) a constancy in numbers of cellular elements is the 

 primitive condition of development which, as Conklin points out, 

 is retained only in the early stages of ontogeny of the metazoa, 

 or (2) cell constancy is a manifestation of a tendency toward 

 fixity which is acquired only at a late stage in the development 

 of a race. 



That there is a tendency for the same number of embryonic 

 cells to be used in the formation of the organs and structures of 



