No. 3-] SEGREGATION OF THE SEX-CELLS. 487 



the two-protovertebras stage. But two of the cells, in the em- 

 bryos examined, are now found in the cephalic region, one on 

 either side a short distance posterior to the origin of the chorda. 

 These cells may correspond to those two seen in the ear region 

 in the stage having but two protovertebrae. A small cell 

 which may belong to this series lies in the tissue of the future 

 heart. The remainder are distributed as follows : one below 

 the seventh sinistral protovertebra ; three in the left side of the 

 tail, i.e. in the region in which protovertebrae have not yet ap- 

 peared ; and three in the right side of the tail. 



The cells in this stage stain deeper and much more uni- 

 formly than the surrounding cells with Grenacher's haematoxylin. 

 They greatly resemble the very early conditions of these cells, 

 and the number would seem to indicate that there has been no 

 segmentation since the two-protovertebras stage. In other 

 larvae of the same stage there are ten, eight, five, and nine cells, 

 respectively. 



In larvae about 0.62 mm. long, the general distribution of the 

 preceding stage is maintained. The cells in the gill region, 

 however, are seen to vary both in number and position. In five 

 embryos examined they are, in detail : — 



No. 1. None in gill region ; one doubtful cell in the heart. 



No. 2. One on the right side of the first gill-slit region ; 

 another (or two ?) on the left. 



No. 3. Three in the region of the right gill ; none on the 

 other side. 



No. 4. One on each side in the gill region, that of the left 

 side inconspicuous (or absent ?). 



No. 5. Two dextral, somewhat further forward than usual, 

 the inner one apparently penetrating the brain. 



There is also apparently one or two cells near the middle of 

 the body, as in the preceding stage. These are in the five em- 

 bryos as follows : — 



No. 1. One in the sixth protovertebra, near its dorsal wall 

 (not distinct). 



No. 2. One dextral, about 0.36 mm. from the posterior end. 



No. 3. One sinistral, 0.25 mm. from the posterior end ; another 

 20 fx further forward on the right. 



No. 4. One in the fourth protovertebra of the right side, and 

 another below the end of the fifth. 



