The Embryology of Chlaynydoselachus 589 



[Egg] C [Figure 4, plate I]. Blastula shown in drawing (44. mm.) round. Shows segtn. 

 over entire surface [of germinal area ?] — margin not good but at several places good transition 

 from marginal blastomeres into central bl'ms. Not possible to trace furrows far down side of 



egg- 



[Egg] B [Figure 5, plate I]. 81. somewhat later than C, [germinal area 38 x 39 mm.]. 

 Margl. blast, fine — surface blast, smaller and less conspicuous. At several points of marg. 

 there are certain irregular folds of which none are (surely) gastrulation erscheinungen [mani- 

 festations]. 



[Egg] A [Figure 6, plate I]. Gastrula 44 x 48 mm. — drawn. No surface markings — 

 except at margins as shn. in fig. [a pencil sketch in outline sectional view accompanies this 

 note] — these continued sometimes over the marg. of yolk. 



This is what Dean wrote and what is portrayed (half the original size) in Figures 4, 

 5, and 6, plate I. Egg A (the circle measuring 44 x 48 mm.). Dean thought to be a gastrula. 

 Eggs B (circle 38 x 39 mm.) and C (circle, 44 x 44 mm.), he calls blastulae. If A, having the 

 largest circle, is a gastrula, then, for all that I can see from the evidence at hand, B and C 

 are not blastulae but gastrulae. Just here note that in C the germinal area (delimited by 

 a shallow circular groove) is asymmetrically placed on the yolk. This asymmetry is 

 like that noted for the third egg shown in Text'figure 26. Furthermore, it may be well 

 to recall here that this is what I have found to be the general rule in the large eggs of 

 G inglymostoma. 



Each egg, as represented in Figures 4, 5, and 6, plate I, has the germinal area marked 

 off by a faint ring apparently representing a shallow circular groove. A similar groove 

 bounds the margin of the germinal area in the preserved, half mature ovarian eggs examined 

 and described earlier in this article. It is quite likely that this groove, if present in the 

 living egg, would persist through cleavage and gastrula stages; or, if it is a fixation artifact, 

 the same conditions would produce it in these stages. Balfour (1885, p. 222) found such 

 grooves in living and sectioned elasmobranch eggs studied by him. 



In these drawings (Figures 4, 5, and 6, plate I), the presence of the germinal area is 

 indicated by faint circles only slightly darker than the remainder of the upper surfaces of 

 the eggs. Indeed in eggs B and C, on one side the circle, as drawn, is so faint that it 

 disappears into the general upper surface of each egg (especially B). Take away the 

 circles and there would be nothing to indicate any germinal area. For another reason it 

 must be noted here that one side of the ring is drawn more heavily shaded than the other. 

 Balfour (1885, p. 225) noted this on his preserved material — "In sections of the germinal 

 disc [of Pristiurus], the groove which separates it from the yolk is well marked on one 

 side, but hardly visible at the other extremity of the section". What then did Dean's 

 artist draw and how did he see anything to draw? If he drew preserved eggs, as is most 

 likely, he drew the thickened edges of the blastoderms in early gastrula stages, more 

 thickened on one edge than the other. This Balfour found as cited above and shows in 

 a cross section of the blastoderm. These dark parts of the circles (Figures 4, 5, 6, plate I), 

 I take to be the edges of the blastoderms wherein the embryos will be found later. In 

 fixed eggs the thickened edges of the late blastoderms would show up more opaque than 

 the inner and thinner regions. 



