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segmented mesoblast and applied to it, also on the top of the yolk-sac, 

 there are some few in the definitive hypoblast, and several in the 

 extra-embryonic portion of this layer. Here and there one is loosely 

 attached to the inner side of the mesoblast, lying really within what 

 in Raja has been termed the germinal path. Further back they be- 

 come more numerous in the extra - embryonic hypoblast and in the 

 upper part of the yolk. The mesoblast now joins the hypoblast, the 

 tail-fold begins, and the germ -cells cease in the few final sections of 

 the embryo. 



Before me lies a combination - figure of the germ -cells in five 

 sections of one row of such from this embryo. In the section outlined 

 there are six germ -cells, and to these have been added seven others 

 in curious places in four neighbouring sections. Of the thirteen germ- 

 cells of the figure three are possibly upon the site of the future ger- 

 minal nidus, four others are much lower down, two are in the hypo- 

 blast, and the remaining four are in the yolk. 



Judging by the finds in embryos nos. 4 and 12 the germ-path of 

 Pristiurus is not so well defined a way as that of Raja, and the great 

 majority of the germ-cells would seem to get into the mesoblast di- 

 rectly from the extra-embryonic hypoblast and yolk, and in this way 

 they may be carried upwards into the germinal nidus. But evidences 

 of amoeboid wanderings abound in both embryos. 



As an example of the variations observable in early embryos of 

 this and other forms the following embryo and its germ-cells may be 

 given. Pristiurus no. 5 is about 2 mm, and, therefore, of about the 

 same age as the preceding embryo. In this instance only two germ- 

 cells were detected in the site of the future germinal nidus, there 

 was one in the germinal path, and five others were seen in other 

 places. The few germ- cells noted here outside the embryonic found- 

 ation is accounted for by the small portion of the blastoderm pre- 

 served. 



The two embryos nos. 4 and 5 belong to very much the like 

 period of the development, that is, they are of about the same age, 

 and both show the wanderings of the germ -cells into the embryo. 

 No. 5 exhibits, possibly, a slightly younger phase of this than no. 4. 

 The small amount of the blastoderm present in no. 5 prevents, how- 

 ever, any very close comparison between them. 



Finally, in Pristiurus no. 2, which is of the earliest period of 

 embryo -formation, and shortly after the close of the cleavage, there 

 are under the blastoderm little heaps of cells of the sizes and cha- 

 racters of the future primary germ-cells. 



