1896.] Sex Cells in Cytnatogaster. 267 



There is no doubt concerning the continuity of the repro- 

 ductive cells in Cymatogaster ; they may be followed from 

 very early conditions till sexual maturity without once losing 

 their identity. No somatic cells are transformed into repro- 

 ductive cells, and the comparative constancy of the number 

 of the latter present in any embryo up to 7 mm. long makes 

 it probable that none 4 are ever changed into any other struc- 

 ture. These statements .apply with equal force to other tissues. 



The difference between the reproductive and the somatic 

 cells is that the latter, after development has begun, continue 

 to develop, divide, grow and adapt themselves to their new 

 duties without intermission. The sex cells, on the other hand, 

 stop dividing at a certain point and remain at apparent rest 

 for a long period. Owing to this arrest in division the sex 

 cells soon stand out prominently as large cells among the 

 smaller somatic cells. Such an arrest in segmentation has 

 been observed in a number of other animals in which the re- 

 productive cells are early segregated, and it cannot be without 

 meaning. It has been supposed that during such periods of 

 apparent rest the cells remain dormant, retaining their em- 

 bryonic character unchanged. I do not think this is the true 

 reason for the difference of development between the soma and 

 the reproductive cells. The reason seems to me to lie in the 

 fact that the sexual organs are the last to become functional, 

 and their development is consequently retarded. The sex 

 cells, when first segregated — that is, when they first lag behind 

 in segmentation — are not exactly like the ovum from which 

 they have been derived, and there is just as true histogenesis 

 in their development into the reproductive tissues as in the 

 case of any other embryonic cells into their corresponding tis- 

 sue. Even during the long period of rest from segmentation, 

 the process of tissue differentiation produces a visible and 

 measurable change. But the difference between embryonic 

 cells and undifferentiated reproductive cells being small, the 

 histogenic changes in them during early stages is correspond- 

 ingly small. This small change has been supposed to amount 

 to no change, and has given rise to that fascinating " myth * the 



4 For possible exceptions see Eigenmann, Journ. Morph., V, No. 3, 1891. 



