748 Bashford Dean Memorial Volume 



these problematical features, the figure shows a moderate advance in pigmentation and 

 a slight increase in the size of the pectoral and first dorsal fins. It is of interest principally 

 because it is the most advanced embryo figured with a yolk sac. 



The stage at hatching, portrayed in Figure 83, plate VII, is discussed in a later section 

 of this article. The embryo represented in Figure 84, same plate, is two weeks older and 

 therefore the consideration of this figure is likewise deferred. 



THE VITELLINE CIRCULATION 



Since the vitelline circulation in Heterodontus japonicus is essentially Like that of 

 other sharks, the diagrammatic figures by Balfour (1885) representing stages in the de' 



Text-figure 58. 



Diagrammatic figures showing the development of the vitelUne circulation on the egg of 



Pristiurus: A, early; B, intermediate; and C, advanced stages. 



a, vitelline artery; v, vitelline vein; y\, yolk blastopore. The letter y (in C) marks the spot where the venous 



ring and the yolk blastopore were closed bythe growth of the blastoderm. 



After Balfour, 1885, Figs. 1, 2, and 3, pi. 9. 



velopment of the vitelline vessels of Pristiurus will serve as an introduction to the study 

 of Dean's drawings. Three of Balfour's figures are reproduced as my Text-figure 58. 



A single unpaired vitelline artery emerges from the yolk stalk and proceeds cephalad 

 along the blastoderm under cover of the head of the embryo (Text-figure 58a). This 

 arterial trunk divides to form two arcuate branches that turn toward the posterior margin 

 of the blastoderm. In the stage represented in Text-figure 58b, the blastoderm has over- 

 grown the entire surface of the yolk mass excepting a small nearly circular area (y}{, the 

 yolk blastopore) posterior to the yolk stalk. The two main arterial branches have almost 

 surrounded the yolk blastopore, but are situated at some distance from it. Numerous 

 small secondary branches grow toward the yolk blastopore, and some of these connect 



