212 



During this period, cell division is also going on, but much less 

 actively, in the more centrally situated part of the yolk hemisphere. 

 In Fig. 1 is shown a sketch made, with the upright microscope, 

 from the living egg compressed in such a position that its side was 

 turned up. The blastopore lip (d. I.) has just appeared. On the 

 opposite side of the egg, four cells are shown. One of these (b) lies 

 well in the randzone, and measures 40 /n X QO /n; one (c) lies at the 

 yolk edge of the randzone, and measures 80 /.i X 8Ö /li. Of the 

 others , cell a , 40 /< in diameter , is a representative cell of its 

 region, and cell d, 100 /< in diameter, is a representative yolk cell. 

 The four cells drawn are typical cells of their respective regions. 

 Cell c, which at this time, is properly designated a yolk cell, was 

 later on incorporated in the ectoderm. The incorporation of such a 

 cell, a process which I have repeatedly watched, and which is 



sufficiently illustrated by Figs. 1 



and 2, proceeds as follows. In the 



/ x mareinal yolk cells, hitherto white, 



/ -nA.^-.j ^ brownish spot more or less central 



/ "^ \ in position appears (due, as sections 



{ - - (' indicate, to pigment granules accu- 



\ ^^ '^: 1 '^ mulating round the nucleus), and 



j^^^ j I the cell body ac- 



\ 0"""/ " r^ [S> (3 q^^^'^s the merest 



\,^^^ ^y^ ^ 1^ trace of brown. 



-— "^ These features make 



Fiff. 1. Fig 2. , . 



their appearance 

 when the cell is 80—100 /< in diameter, the central accumulation of 

 pigment (chromatophore as it may be called) often being noticeable 

 some time before any trace of color can be observed in the rest of 

 the cell body. The chromatophore increases in size, becoming V4 

 to ^/g diameter of the cell, and also grows darker. Some time 

 after the appearance of the chromatophore (30 mins. in a typical 

 case), it elongates and divides (pigment evidently continues to 

 surround the dividing nucleus), the division of the cytoplasm taking 

 place shortly afterwards. The resulting cells measure about 60 /t 

 in greatest diameter, have relatively larger and darker chromato- 

 phores than the mother cell, and clearly belong to the light brown 

 randzone. A second cell division, likewise preceded by division of 

 the chromatophore, follows soon after the first — many of the cells 

 in a region dividing simultaneously. In the resulting cells which 

 measure for the most part 40 ,a in diameter, the pigmentation 



