PLATE V 



ENTIRE EGGS OF HETERODOHTUS JAPOHICUS 



SHOWING CLOSURE OF YOLK BLASTOPORE 



AND ORIGIN OF VITELLINE VESSELS 



Fig. 52. The blastoderm covers aiore than a hemisphere of the yolk mass. The stage of embryonic devel- 

 opment is perhaps equivalent to that shown in Figure 26, plate II. 



Fig. 53. In this figure the yolk blastopore is appreciably smaller, though the embryo is no larger than the 

 one represented in the preceding figure. 



Fig. 54. Here the embryo appears to be in a stage slightly younger than the one shown in Figure 53, 

 though the yolk blastopore is smaller. 



Fig. 55. In this figure the embryo is in a stage approximately the same as the preceding. 



Fig. 56. This is the first stage showing vitelline vessels, here entirely arterial. The main arterial trunk is 

 not visible. There are two pairs of secondary vitelline arteries, right and left. The embryo is 

 larger than the one shown in the preceding figure. 



Fig. 57. This figure shows a decided increase in the size of the embryo and in the degree of closure of the 

 blastopore. There are two vitelline arteries on the left, but only one on the right. A venous 

 ring, surrounding the yolk blastopore, is in process of formation. 



Fig. 58. The main trunk of the vitelline artery is shown proceeding forward from the yolk stalk, and 

 branching to form only one pair of arcuate arteries. A multitude of radially-directed vitelline 

 venules drain into the venous ring. 



Fig. 59. The yolk blastopore is nearly closed. There is no change in the arterial pattern, but the venules 

 are further developed. The embryo is in approximately the same stage as the one represented in 

 Figure 29, plate II. 



Fig. 60. The venous ring has contracted almost to the point of disappearance. The arterial pattern is 

 unchanged. The embryo is decidedly larger than the one shown in the preceding figure. 



Fig. 61. In this figure the vitelline vessels are especially well shown. Right and left vitelline veins extend 

 to the margin of the figure. There are two main branches of the vitelline vein, extending from 

 the plexiform group of venules to the yolk stalk. 



Fig. 62. An egg with two embryos (identical twins), perhaps with a single tail. Each embryo has its own 

 vitelline artery and vein, but the veins drain the same nexus of venules. 



Fig. 63. The arterial vitelline trunk passes unbranched to the other side of the egg. There is a single 

 vitelline vein draining a dendritic group of venules. The embryo still heads in the direction of 

 the vitelline artery. 



Figures 52 to 59 show stages in the closure of the yolk blastopore. In Figures 52 to 56 the problematical "cleavage" 

 furrows of the yolk are conspicuous in the yolk blastopore. In Figures 53 and 54 some of these furrows show faintly, 

 and in Figures 58 and 59 many of them show conspicuously through the translucent blastoderm. 



Figures 56 to 63 show early stages in the development of the vitelline vessels. In some of these drawings, the pattern 

 of the vessels is more or less obscured by the presence of "cleavage" furrows in the yolk, which show through the 

 blastoderm. 



All these drawings are in natural colors save that the embryos, which in life are colorless and translucent, are portrayed 

 in opaque white. With the possible exception of Figures 58, 59 and 60, all the drawings are here reproduced about 

 natural size. 



