CELL CONSTANCY IN THE GENUS EORHYNCHUS 287 



morphologically of more importance, while the later divisions in 

 .Crepidula are believed to be non-differential and inconstant, but 

 so far no one has offered a satisfactory explanation of why this 

 is true. I do not here attempt to suggest any new factors as de- 

 termining the number and arrangement of the cleavages involved 

 in organ formation, but from the facts observed in the develop- 

 ment of colonial protozoa of constant numbers, and in the on-' 

 togeny of the metazoa where constancy has been demonstrated, 

 there must be some inherent factor determining that a certain 

 number of cell divisions shall precede the formation of a given 

 organ, and that when this number is realized the power of further 

 nuclear division becomes lost. Moreover evidence gained from 

 the study of the members of the genus Eorhynchus tends to dis- 

 credit the possibility of cell size acting as such a factor, for in 

 case size determines the time when a cleavage or division takes 

 place, cell division would continue throughout the life of the 

 individual, since these worms continue to increase in size from 

 the time they reach the final host until they are expelled from it. 

 There must be some factor within the original cell which deter- 

 mines every cell division, from the first cleavage to the establish- 

 ment of the final adult condition. Whether this determining 

 element is a 'force' which becomes dissipated in the processes 

 of cleavage so that, when the definitive number of cells has. been 

 attained, no further progress is possible on the account of the lack 

 of this 'force,' or whether the cause is to be sought in the pres- 

 ence of definite substances within the cell in the nature of either 

 'determiners' regulating cleavage or materials that are used up 

 in the processes of mitosis can not be decided in the present stage 

 of knowledge of the subject. 



4. Cytomorphosis 



In the genus Eorhynchus is found an evident exception to the 

 generally accepted view of celluhar changes accompanying ad- 

 vance of age in animal tissues. Probably the most important 

 point of difference, or at least the one most easily observed, 

 is the fact that as soon as the animal has reached the point 



