475 



pathway is directly upwards from the yolk-sac into the embryo. If 

 they take the wroog path, they may finally arrive almost anywhere. 



The germ-path is a very definite one. It is from the yolk-sac 

 upwards between splanchnopleure and gut in the hinder portion of 

 the blastoderm. It is here that in early stages (6 to 8 mm embryos 

 and even in larger ones) a great many of them are encountered in 

 various positions (figs. 24, 26, 29 and 36). 



This pathway, which may, therefore, be termed the germinal 

 path^), leads them directly to the position, which they ought finally 

 to take up in the "germinal ridge" or nidus. 



That this is a definite track is shown 1) by the great number 

 of germ-cells in it in early stages, and 2) by the very large number 

 — the majority in fact — which have either gone to its end, or halted 

 somewhere along it. Their instinct appears to be to go along this 

 path, and then to pierce the splanchnopleure. Those which go to the 

 end and those which tarry carry out these "instructions". The former, 

 in embryo no. 454 numbering 349, reach the germinal nidus; the 

 latter, in the same embryo 110, find a resting place somewhere on 

 the mesentery to the ventral side of the nidus, and, it may be, as 

 low down as the subintestinal vein, or even on the opposite wall. As 

 already seen, in embryo no. 454 only some 53 probably failed to 

 follow the germinal path ^). 



The .wanderings of some of the germ-cells would appear to con- 

 tinue for a relatively long period of the development. At any rate, 

 they are met with between splanchnopleure and gut, as well as in 

 other places, in embryos of 14 to 17 mm, or even of 20 mm. For 

 some time after this latter period, apart from those which have got 

 into impossible places, one meets with amoeboid germ-cells only in 

 the connective tissue above and around the gut. The gut is now shut 

 otf from the yolk-sac, and to this period dates the first appearance 

 of a narrow yolk-stalk, connecting embryo and yolk-sac. That is to 

 say, the embryo is now well raised up from the yolk-sac. 



At least until the embryo is 42 mm the primary germ-cells remain 



1) Originally written "germinal track", but this term has beem 

 employed by Weismann in a very different sense. ' 



2) The above is given as a typical example, other cases will bft 

 found in the descriptive part of the memoir. It may be added, that 

 in all probability the final definite number of normally placed primary 

 germ-cells is subject to variations. Moreover, germ-cells may degenerate 

 at any period of the life-history. These variations are sufficient to 

 account for wide differences in the virility of different individuals. 



