No. 3.] SEGREGATION- OF THE SEX-CELLS. 485 



the cells of an egg undergoing the ninth segmentation and in 

 all probability it is a cell remaining unchanged from that stage. 

 It contains yolk particles. Most of the sex-cells are collected 

 in a limited region at this stage in the thickened portion of the 

 embryo, where the three germ layers fuse. This would lend 

 force to the supposition that they are derived from two cells at 

 most — one dextral and one sinistral. There are a few scattered 

 cells in other parts of the embryo which cannot be so derived 

 unless they early migrate from their original position. 



There are, on an average, thirteen sex-cells in an egg of this 

 stage. The largest number noticed is seventeen, the smallest 

 nine. These, in an egg containing fifteen, which was cut into 

 thirty sections, the first of which is tangential to the thickened 

 part of the embryo, are distributed as follows : Section 5 has 

 two cells, section 6 (Fig. 4) has seven, section 7 has three (Fig. 

 4 a), section 13 has two, and section 14 has one lying in the 

 mesoderm on the side of the egg opposite the neural thicken- 

 ing. Twelve of the fifteen sex-cells lie in the thickened portion 

 of the embryo where the germ layers are fused. The one lying 

 in the ventral part of the mesoderm is stained slightly darker 

 than the others. The staining varies, however, in the collected 

 ones, as, indeed, does the staining of individual cells of the 

 blastoderm. 



In an egg with fifteen sex-cells there are nine in the thickened 

 portion, five in close proximity to it, and a ventral one. 



In another there are fourteen inside the thickened region, 

 two outside, and one ventrally placed. In the other eggs I can- 

 not find any ventral cell. 



The position and character of the sex-cells in the stage with 

 two well-formed, and one or two outlined protovertebrae is gath- 

 ered from three eggs killed with Flemming's strong solution of 

 osmic, chromic, and acetic acid, and stained with Delafield's 

 haematoxylin. This method at times differentiates the sex-cells 

 from the surrounding ones very clearly. 1 The sex-cells in some 

 larvae are stained dark brownish gray, while the remaining cells 

 are stained bright blue — especially the nucleolus and the proto- 



1 A large number of lame were treated in this way, and though they were car- 

 ried through the whole process from killing to sectioning together, the results were 

 so disastrously different that this method cannot be depended upon to always give 

 satisfactory results. 



