628 John bearö, 



For the information of embryologists, who are better acquainted 

 with ScyUium canicula than with Baja hatis, it may be added, that 

 the above embryo corresponds to 2 embryos of ScyUium canicula 

 (Nos. 116 and 118) in the majority of its characters. These measured 

 respectively 18 and 18.5 mm in the preserved condition. 



It has not been determined in what segment of the trunk the 

 first normally-placed germ-cells make their appearance. Rabl ('96) 

 records them as commencing in early Pristiurus-emhry os, (embryos of 

 100 somites) in the region of the 12th trunk-segment. In embryo 

 No. 454 they begin, and are at first not very numerous, some little 

 way behind the front end of the segmental duct. Very soon they are 

 found to be situated in or upon a well-marked ridge, the "germinal 

 epithelium" of various authors from Waldeyer ('71) onwards. 



Only those germ-cells will be described as normally placed, which 

 are in, on, or within this ridge. The "germinal ridge" of this em- 

 bryo nowhere attains a greater lateral extension, and nowhere does 

 it project more into the body-cavity than in Fig. 37. Posteriorly, 

 the ridge on each side flattens out. Further back, it here and there 

 projects slightly, and the epithelium extends backwards as far as 

 the cloaca. 



Almost throughout its whole course there are germ-cells connected 

 with the ridge. They one and all closely resemble those shown in 

 Fig. 37, to be afterwards described. 



My notes contain tables, spread over five large pages, showing 

 the distribution of both normal and "abnormal" germ-cells in this 

 embryo. In the following these tables will be condensed, and in 

 Table 1 a summary of the counting will be found. The number of 

 rows of sections, in which germ-cells occur, is 23. The number of 

 sections in each row is not always the same ; but, owing to gradual 

 decrease in the area of the sections, it increases from an average of 

 15 to one of 29. 



504 is the maximum number of germ-cells in an undegenerate 

 condition in the present embryo No. 454. 



Owing to two circumstances, the comparative thickness of the 

 sections and the marked contrast presented by germ-cells to all the 

 other non-yolk-containing cells of the embryo, it has been possible to 

 count the germ-cells with a considerable degree of accuracy. This is 

 not so easy a task as one might imagine. Many of them are cut in 

 section; and, when one has to deal with three or four close together 



