MIGRATION OF SEX-CELLS OF FUNDULUS HETEROCLITUS. 327 



was of course recorded. Most of the observations were made 

 from serial transverse sections because they show the dorso- 

 ventral position of the sex-cells more clearly in relation to the 

 outstanding features of the developing embryo than do those cut 

 longitudinally. 



OBSERVATIONS. 

 Criteria of the Primary Sc.r-cclls. 



The enumeration of criteria for any group of cells as distin- 

 guished from all others in a series of embryos is a task which 

 promises but doubtful results. There can be no question however 

 that the primary sex-cells do have distinctive characteristics which 

 make them easily recognizable, during the resting stages, to one 

 who has had them under observation. It is not always feasible 

 positively to identify the cells during division. 



Throughout the migration period these cells maintain the same 

 general characteristics. There are, to be sure, slight variations in 

 the ratio between the nuclear and cytoplasmic elements, in size 

 and in the character and arrangement of the chromatin granules ; 

 but these features may be observed only on close inspection, 

 rather than in a preliminary study of the primary sex-cells. 



The primary sex-cells vary in diameter from 9 to 128 micra. 

 As contrasted with other cells they are spherical or ovoid with 

 very definite cell outlines. The nuclei conform to the general 

 shape of the cell body within which they are located. The cyto- 

 plasmic content is always clearer and takes less stain than that of 

 the surrounding cells. Likewise the achromatin of the nuclei is 

 quite clear, allowing the chromatic granules to stand out in bold 

 contrast. The limn network is directly beneath the nuclear mem- 

 brane, and due to this arrangement the chromatin granules are 

 distributed peripherally over the nucleus. This peripheral ar- 

 rangement of the chromatin is a constant distinguishing charac- 

 teristic not to be mistaken, for it is never produced in any other 

 cells. The linin network is connected to one, or more frequently 

 to two nucleoli which are located near the center of the nucleus. 

 No peculiar invagination of the nuclear membrane, such as was 

 reported by Dodds (1910), was observed in Fundnhis. An un- 

 usually large centrosome is, as a rule plainly visible in the cyto- 



