650 JOHN BEARD, 



noted in a great many cases, more especially in the earlier embryos 

 of 10 mm and under. Later on it seems as a rule to disappear, 

 although, as will presently appear, it may crop up again in a new 

 form. There is only one figure (Fig. 10) showing it in the plates: 

 most of the figures having been drawn before the phenomenon was 

 noticed. Moreover, it will be obvious, that the duplication will be 

 visible only when the section passes through the nucleus in a certain 

 plane. The discovery, which there need be no hesitation in identify- 

 ing with the finds of the aforesaid investigators, also served to throw 

 light on certain figures of degenerating germ-cells, previously drawn. 

 These are Figs. 38 to 47 on PI. 44. They are described in a sub- 

 sequent section. Here it need only be noted, that they serve to 

 show an independence of the paternal and maternal parts of the 

 nucleus in degenerating germ-cells from an embryo of 32 mm. 



Among my sections there are nearly a dozen sets from the seg- 

 mentation. Of these the two, which immediately precede the formation 

 of the embryo, have been of service. They are of dilferent ages. The 

 one (No. 422) is of the period, just prior to the so-called invagination; 

 the other (No. 434) is just after its start. 



Even in the more extensive material of Scyllium of about this 

 period I have never been able to convince myself, that the "tucking- 

 in" at the posterior end of the blastoderm in Elasmobranchs is com- 

 parable to an invagination. One might with more propriety term the 

 tucked-in portion of the blastodermic rim a "primitive streak". It is 

 a growing zone, made up of actively dividing cells, from which cer- 

 tain parts of the future embryo are being proliferated forward. As 

 is well known, if one except a hypoblastic portion, from which the 

 proliferation is taking place, these parts are what is usually termed 

 mesoderm ^). 



In longitudinal section it is seen, that the roof of the blastoderm 

 is composed of a columnar epithelium. Anteriorly, there is a large 

 mass of cleavage-cells, laded with yolk. Posteriorly, there is a roomy 

 "germinal cavity" under the region of the future embryo. Under- 



1) It would take up too much space and time to quote here various 

 memoirs on the mesoderm of Elasmobranchs, or Rabl's immense "Theorie 

 des Mesoderms". The writer is not unacquainted with these, but the 

 present writing is not intended to be in any sense a contribution to a 

 knowledge of the "mesoderm". 



