393 



During this period, the sex-cells are migrating from the edge of the 

 area pellucida to their final positions in the sex-glands, mesentery, 

 alimentary tract, and adrenal region. During this migration, the sex- 

 cells are massed together to such an extent as to make it in all cases 

 difficult, and in some cases impossible, to follow any given cell from 

 one section to another. Attention was directed to the nucleus in 

 counting. Since the sections were thin, the nucleus was cut in the 

 majority of cases. If the section of the nucleus were thin, the deeply 

 stained yolk spherules of the cell body masked it; while if it were 

 in evidence, the thinner section of any given nucleus could usually be 

 recognized and omitted in the count. In later stages, the sex-cells 

 are so scattered as to make precision in counting possible. In these 

 stages, great pains were taken to avoid counting the same cell twice, 

 adjacent sections being repeatedly compared. I feel confident that the 

 count in these later stages would come to within 25 of the actual 

 number for the entire embryo. In the earlier stages, the error might 

 in extreme cases, reach 100, but in all cases, the chances are that 

 the numbers here given are not very far from the correct ones. 



\ 



Table showing the number and arrangement of sex-cells in embryos 

 varying in size from a total length of 2,8 mm to a length of 5 mm as 

 measured from the cervical bend to the tail bend (c-t). During this 

 period, the sex-cells are migrating in the endoderm from the edge of 

 the area pellucida to a position beneath the mesenter}' : 



I 



Cells classed as "indeterminate" were found behind the embryo 

 more or less close to the median axis; hence it was not possible to 

 determine to which side they belonged. Brackets indicate that included 

 embrvos were taken from the same nest. 



