The Embryology of Heterodontus japonicus 739 



down the definitive entoderm. In later stages, circumcrescence provides a protective and 

 vascular covering for the yolk mass. These extra'embryonic structures will be 

 eventually resorbed. 



The yolk blastopore of Heterodontus japonicus is probably unique in that the surface 

 of the yolk is traversed by furrows that appear to be cleavage furrows. These are clearly 

 shown in Figures 46 to 56, plates IV and V. They are distinct in some of Dean's photo- 

 graphs, both published and unpublished (c/. page 727). In Figures 52 to 56, plate V, the 

 appearance of the yolk blastopore is strikingly like that of the yolk plug of some amphibian 

 eggs (e.g., Cryptohranchus as described by Smith, 1912, Figs. 115 and 138 to 140). In both 

 cases the yolk is traversed by furrows, and the pattern is much the same. It is obvious 

 that in nearly all the drawings of developing eggs of H. japonicus, in stages from the 

 beginning of gastrulation until after the closure of the yolk blastopore (Figures 46 to 61, 

 plates IV and V), some of the problematical cleavage furrows are visible even after they 

 have been covered by the translucent blastoderm. 



The closing phases of the yolk blastopore (Figures 57 to 61, plate V) are marked by 

 variability in its outlines and by the presence of the vitelline vessels (considered in a later 

 section). Figure 61, plate V, represents the final stage in the closure of the blastopore. 

 The site of closure is not far behind the yolk stalk. 



In a portion of his manuscript already quoted in my section on ''General Mode of 

 Development", Dean states that the blastoderm of Heterodontus japonicus "still grows 

 around the egg before the embryo is of large si2;e" and seems to regard this as a character of 

 considerable phylogenetic importance. This view implies that in more modern sharks 

 closure of the yolk blastopore is delayed. Some measurements of the siz;e of the early 

 embryo in relation to the si2;e of the entire blastoderm have a bearing on the problem. 



The advanced gastrula of Heterodontus japonicus represented in surface view in 

 Figure 13, plate I, has a blastoderm about ten times as long as the embryo proper, which is 

 about one millimeter long; while a gastrula of Torpedo ocellatus in about the same stage, 

 drawn by Ziegler (1892, Text-fig. 3) in surface view, has a blastoderm only five times its 

 length. A similar comparison may be made in median sagittal sections. Dean's gastrula 

 represented in my Text-figure 56 has a blastoderm (extra-embryonic portion not shown in 

 the drawing) about eight times as long as the embryo proper; while Ziegler's figure (1892, 

 Fig. 15, Taf. Ill) of a gastrula of Torpedo ocellatus in approximately the same stage has 

 a blastoderm (drawn entire) about three and one-half times the length of the embryo 

 proper. These measurements indicate that the blastoderm of Heterodontus japonicus 

 grows faster, or at least spreads out more rapidly, in proportion to the si2;e of the embryo, 

 than does the blastoderm of Torpedo ocellatus. 



There remains the question concerning the comparative sizes or stages of develop- 

 ment of embryos at the time when overgrowth of the yolk by the blastoderm is completed. 

 The egg (yolk mass) of H. japonicus in the early stages of embryonic development measures 

 from 40 to 50 mm. in diameter. The egg depicted in Figure 61, plate V, is in the stage in 

 which the yolk blastopore has just closed. As compared with the other eggs in approxi- 



