THE KORMATION OF (JKk.M-CKLLS 



193 



in most of the higher Metazoa this only occurs after the forma- 

 tion of hundreds or thousands of cell-generations. 



The position of the germ-track may also vary. In the Diptera 

 it is quite distinct from the somatic cell-tracks, and the <renea- 

 logical trees of these two kinds of cells separate at the root. In 

 the Dap/inidcE the germ-track passes through each of the first four 

 segmentation-cells, and then branches ofl" from the somatic 



Fig. 13. — Three early stages in the development of Sagitta. — (After ( ). Heriwiy.; 

 In A the differentiation of two primitive germ-cells (g) from the endoderm, and 

 in B and C the multiplication and separation of these cells is shown. (From 

 Lang's ' Lehrbuch der vergleichenden Anatomic.') 



tracks. In Sagitta it passes through the primary endoderm 

 cell, and the primary germ-cells separate from the primary 

 endoderm before the definitive endoderm of the alimentary canal 

 has been formed from the latter. In RJiabditis )ii^rove}iosa the 

 germ-track extends through three generations of endoderm cells, 

 passes into the primary mesoderm, and after several generations 

 branches off from two of the mesoderm cells. In most of the 

 Metazoa, however, the formation of the primary germ-cells is 

 postponed to a still later period, so that the separation of the 

 germ-branch from the somatic branch takes place at a much 

 higher level on the genealogical tree of the cells, and often first 

 occursin the younger and smaller lateral branches. The primary 

 germ-cells do not always branch off from the track of the endo- 

 derm, but may just as often diverge from that of the ectoderm. 

 In the lower Medusae, for instance, in which the development is 

 a direct one, the germ-cells become differentiated at a very late 

 stage from the ectoderm cells of the body, which is already 

 fully formed and often independent and self-supporting ; while 

 in the higher Medusae and Ctenophora the primary germ-cells 

 are derived from the endoderm. We thus see that the germ- 

 tracks or series of cells which lead from the egg-cell to the 



