MIGRATION OF SEX-CELLS OF FUXDULUf, HETEROCLITUS. 329 



1920) and others, is a question which may be omitted from the 

 present discussion. 



Observations of the Germinal Path. 



From many available embryos the following were selected for 

 consideration because they constitute representative stages, and 

 are essential to a clear understanding of the migration of the 

 primary sex-cells. 



Embryos from 46 to 50 Hours. Three embryos of this very 

 early stage, designated in our material as B' 2iX, B' 2iL and 

 B' 23 respectively were carefully studied. Others were available 

 but they were used merely as checks on the three which are 

 reported. 



The position of the sex-cells at this stage is most striking. 

 There is a wide range of distribution in each embryo. The most 

 anterior of the sex-cells \vere invariably farther along the ger- 

 minal path than were the more posterior ones of the same embryo. 

 To demonstrate this fact Fig. 20 has been drawn ; it is an exact 

 outline diagram of B' 2iX, reconstructed by the most accurate 

 means possible. The lateral extent of the neural tube, of the 

 mesoderm and the positions of the sex-cells were determined by 

 measuring from the median line. The thickness 'of the sections 

 was known. These determinations were plotted on millimeter 

 paper and the outline filled in as indicated by the plotted guides. 

 The exact antero-posterior positions of the sex-cells were deter- 

 mined by counting the sections of the serially sectioned embryo. 

 Figs. 9, 10 and 11 are outline drawings from the embryo B' 2iX 

 which was sectioned transversely, and Fig. 12 from B' 2iL which 

 was sectioned longitudinally. These drawings show the exact 

 positions of the sex-cells more clearly than would be possible in 

 a written description. 



The letters A, B and C on Fig. 20 indicate the positions of the 

 sex-cells shown in Figs, n, 10 and 9 respectively. Fig. 11 illus- 

 trates clearly the position of the primary sex-cell in the extra- 

 embryonic region at the posterior of the embryo. Four sex-cells 

 are shown in Fig. 12 as being lateral to the undifferentiated endo- 

 dermal cell mass and ventral to the elongating tail. In Figs. 9 

 and 10 the migration has progressed proportionally to the devel- 



