The Embryology of Chlamydoselachus 



585 



Here, they are merely wrinkles in the very delicate vitelline membrane, probably due to 

 shrinkage of the yolk mass during preservation in the mixture of formalin and alcohol. 

 Unless examined with a lens, they might readily be mistaken for radial cleavage furrows. 

 Some of the lines extend halfway to the equator of the egg. I find that Scammon (1911, 

 Figs. 6 and 7, pi- I) shows similar radial wrinkles outside the blastoderm in early gastrula 

 stages of Squalus acanthias. These I take to be identical with the very fine lines in 

 Dean's sketches. 



The germinal area of the egg of Chlamydoselachus (as outlined by the circles in 

 Figures 4, 5, and 6, plate I) is unusually large. The question arises, how much of this area 

 is occupied by the mass of completely formed blastomeres in the late blastula or early 

 gastrula stages. In his description of egg C, a "blastula". Dean states that it (the germinal 

 area?) shows segmentation over its entire extent. This segmentation might include 

 radial furrows extending beyond the limits of the blastoderm proper. The only drawing 

 which gives a comprehensive picture of the cleavage pattern is the one in the phylogenetic 

 series (Text'figure 22), an equatorial view. In this, the blastoderm proper is not sharply 

 defined. It is evidently larger than that of most elasmobranchs, but decidedly smaller 

 than the germinal area in which it lies. Nishikawa (1898) states that the earliest stage 

 (a blastoderm) that he was able to obtain was nearly circular in form and had a diameter of 

 1.3 mm. He mentions a later blastula, but does not give its size. 



It is therefore clear that the blastoderm, in the narrow sense, occupies only a small 

 central portion of the germinal area. If, during cleavage, the radial furrows extend to, or 

 beyond, the margin of the germinal area, they must be extraordiarily long. I have 

 mentioned the presence, in late ovarian eggs, of fine parallel wrinkles in the vitelline mem^ 

 brane, extending in a meridional direction and simulating cleavage furrows. My ob' 

 servations were made on eggs in preservative for more than thirty years and I have had 

 no opportunity to examine eggs in the blastula stage. I do not know of any other shark, 

 save only Cestracion (Dean, 1901.2), in which the radial cleavage furrows extend so far 

 from the region of completed blastomeres. 



Text-figure 23 

 Section through the margin of the 

 blastoderm of an egg of Chlamydo- 

 selachus in a late blastula stage. 

 This drawing probably represents 

 the thicker end of the blastoderm in 

 the same series used by Dean for the 

 drawing reproduced in my Text' 

 figure 24. 

 After Nishikawa, 1898, p. 97. 



