The Embryology of Heterodontus jdponicus 751 



are not entirely compatible, though they represent different aspects of two eggs in nearly 

 the same stage. This is the first drawing to show the arterial circle, though it may occur 

 in an earlier stage. The arterial circle gives off many branches that reach the margin of the 

 figure, but none of these appear in Figure 64, plate VI, except possibly a few connecting 

 with the viteUine vein. Figure 65 shows the vitelHne artery giving off two side branches, 

 but these are not present in Figure 64. Likewise the venules of the two figures do not 

 correspond. The embryo, as shown by its orientation, is now free to rotate at least 90 

 degrees on the axis of the yolk stalk. 



Figure 66 and 67, plate VI, are companion drawings. The arterial ring shown in 

 Figure 67 is very narrow, and its branches are numerous. Some of the arterioles reach the 

 surface shown in Figure 66; and conversely, some of the venules shown in profusion on 

 this surface interdigitate and probably connect v^ath the arterioles on the opposite side of 

 the egg. There are two vitelline veins, coursing nearly parallel to each other, each with 

 its own system of branches. 



An extreme example of the tendency for the vitelline veins to occur in two groups, 

 right and left, is found in the egg represented in Figure 78, plate VII. The arrangement 

 here reminds one of the condition found in the corresponding stage of Chlamydoselachus 

 (Gudger, 1940, pages 603 and 619; Figure 7, plate I; and Text-figure 4). All the smaller 

 vitelline veins, in Chlamydoselachus, drain into a single median vein; but two groups of 

 veins, right and left, are prominent. 



Figures 68 and 69, plate VI, are companion figures representing different aspects of 

 the same egg — in a stage slightly later than the preceding. There is no essential difference 

 in the pattern of the vitelline vessels save that the arterial circle is nearly closed by the 

 coalescence of segments that have become approximated. The two main branches, right 

 and left, of the vitelline veins unite before reaching the yolk stalk. 



Figures 70 and 71 (companion figures on plate VI) are similar to the preceding with 

 the exception that Figure 71 shows complete absence of the arterial circle. This has been 

 replaced by an extension of the main trunk of the vitelline artery. The portion of the 

 arterial trunk derived from the arterial circle branches profusely, while the stem portion, 

 shown in Figure 70, lacks branches. This figure represents the latest stage in which the 

 patterns of both arteries and veins retain a high degree of bilateral symmetry. 



Figure 72, plate VI, shows little change from the preceding stage save that the veins 

 and venules are more profuse. Its companion. Figure 73, plate VI, exhibits a decided lack 

 of symmetry in both arteries and veins. Its most striking feature is that, on one side, 

 a large number of venules reach almost to the main branches of the vitelline artery. 

 The dark area around the head of the embryo in Figure 72 is probably the optical effect of 

 an oil drop in the yolk mass. 



Figure 74, plate VI, shows around its margin a profuse interdigitation and intercon' 

 nection of arteries and veins. The same situation prevails in the upper portion of Figure 

 75, which shows the reverse side of the same egg. Here, as in the preceding stage, the 

 vitelline artery has two main, though unequal, branches. The large circular dark area 



