EXPLANATION OF PLATE VIIL 



yk. yolk. 



hi. blastoderm. 



a. arteries of yolk sac (red). 



V. veins of yolk sac (blue). 



X. portion of blastodenn outside the arterial circle in which no blood- 

 vessels are present. 



1. Yolk of a Pristiunis egg with blastoderm and embryo. About tv»-<)- 

 thirds of the yolk has been enveloped by the blastoderm. The embryo is 

 still situated at the edge of the blastoderm, but at the end of a bay in' the 

 outline of this. The thickened edge of the blastoderm is indicated by a 

 darker shading. Two arteries have appeared. 



2. Yolk of an older Pristiurus Q%g. The yolk has become all bat 

 enveloped by the blastoderm, and the embryo ceases to lie at the edge of 

 the blastoderm, owing to the coalescence of the two sides of the bav which 

 existed in the earlier stage. The circulation is now largely developed. It 

 consists of an external arterial ring, and an internal venous ring, the latter 

 having been developed in the thickened edge of the blastoderm. Outside 

 the arterial ring no vessels are developed. 



3. The yolk has now become completely enveloped by the blastoderm. 

 The arterial ring has increased in size. The venous ring has vanished, 

 owing to the complete enclosure of the yolk by the bla.stoderm. The point 

 where it existed is still indicated {y) by the brush-like termination of the 

 main venous trmik in a number of small branches. 



4. Diagrammatic projection of the vascular system of the yolk sac 

 of a somewhat older embryo. 



The arterial ring has grown much larger and the portion of the yolk 

 where no vessels exist is very small {x\ The brush-like termination of the 

 venous trunk is still to be noticed. 



The two main trunks (arterial and venous) in reality are in close contact 

 as in fig. 5, and enter the somatic stalk close together. 



The letter a which points to the venous (blue) trunk should be v and 

 not a. 



5. Circulation of the yolk sac of a still older embryo, in which the 

 arterial circle has ceased to exist, owing to the space outside it having 

 become smaller and smaller and finallv vanished. 



