The Embryology of Heterodontus japonicus 749 



with small veins emptying into a venous ring close to the margin of the blastoderm. 

 The main trunk of the vitelline vein drains the venous ring and courses straight to the 

 yolk stalk. 



In the stage shown in Text-figure 58c, the yolk blastopore has been entirely over- 

 grown by the blastoderm. The venous ring has disappeared, and the area formerly 

 occupied by the yolk blastopore is now traversed by a continuation of the main trunk of 

 the vitelline vein. This trunk now receives numerous small veins, usually joining it at 

 right angles. The arterial ring, formed by the two main branches of the vitelline artery, 

 gives off numerous secondary branches which subdivide repeatedly as they grow toward 

 the venules. The arterioles interdigitate with the venules or connect with them presum- 

 ably by means of capillaries. There are no branches extending in a centrifugal direction 

 from the arterial ring. The main arterial trunk does not give off side branches. The later 

 stages in the development of the vitelline vessels of Pristiurus need not be considered. 



Dean's series of figures depicting stages in the development of the vitelline vessels of 

 Heterodontus japonicus is the most extensive and detailed portrayal of these vessels in any 

 Elasmobranch known to the writer. Even the smallest vessels appear to be drawn with 

 great fidelity and accuracy, but owing to their profusion many of them, even in the original 

 drawings, can be distinguished only by using a reading glass. 



In Dean's series of drawings, the first to show vitelline vessels is Figure 56, plate V. 

 In this figure two delicate arteries on each side, branches of a median unpaired vitelline 

 artery, proceed laterad and then caudad. These arteries give off a few short secondary 

 branches barely distinguishable in the drawing. The posterior artery on the right is in 

 process of disappearance, having lost its connection with the main trunk. The thick red 

 ring surrounding the yolk blastopore in this and in earlier stages is not a blood vessel; it 

 is the remains of the pigment of the "orange spot", now confined to the extreme margins 

 of the blastoderm. 



In Figure 57, plate V, there are two branches of the vitelline arterial trunk on the 

 left, but only one on the right; there are only faint indications of secondary branches. 

 A great many very small venules (best seen with a reading glass) drain into the red zjone 

 surrounding the yolk blastopore. Presumably the red 2;one now contains a venous ring 

 or at least a venous network. 



The simple pattern of the arteries pictured in Text'figure 58a is attained, in Het- 

 erodontus japonicus, only after the early developmental irregularities have been smooth- 

 ed out. In Figure 58, plate V, we see such an arterial pattern, but the veins have attained 

 the stage shown in Text-figure 58b. In H. japonicus the venules that drain into the venous 

 ring are very numerous, but they are so slender and set so closely together that individual 

 venules can be made out only with the aid of a reading glass. In the stage represented in 

 Figure 58, plate V, there is an irregular venous ring. This figure, and some of those that 

 follow, are complicated by the presence of the problematical "holoblastic cleavage" 

 pattern, described by Dean and illustrated by Figures 1 to 6, plate I, of this article. These 

 "cleavage" furrows have been overgrown by the blastoderm, but in the living egg they 



