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Bashford Dean Memorial Volume 



Text-figure 30 



Three diagrammatic figures showing the development of the vitelline circulation on the 



egg of Pristiurus. A is a beginning, C an intermediate, and D an advanced stage. 



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



blastopore were closed by the growth of the blastoderm. 



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



seen that the blastoderm has grown over all the yolk save a central area (the so-called yolk 

 blastopore, y}(), forward of which the embryo is found. The two arms of the vitelline 

 artery (a) are in the act of joining behind to form the arterial ring. These arms give off 

 many small arteries on the inside of the hinder half of the ring. Surrounding the yolk 

 blastopore, a venous ring has arisen in the edge of the blastoderm. From its anterior part, 

 there has developed a main venous trunk which reaches to the yolk stalk. The venous 

 ring receives many veinlets on its outer side. 



In Text-figure 30c, the vitelline circulation has made much progress. The arterial 

 ring is complete, has increased in size, and even in the anterior region gives off many small 

 arteries. The yolk blastopore has disappeared, due to the complete enclosure of the yolk 

 by the growing blastoderm. The letter y marks the point of closure of the blastopore. 

 The venous ring has been replaced by the main venous trunk (v) which has grown not 

 only longer but larger as it approaches the yolk stalk. With its many lateral branches, 

 the vitelline venous system much resembles a tree. These veinlets receive blood from the 

 arterioles, and the great venous trunk brings to the growing embryo much blood laden 

 with food stuff. 



With this brief explanation, let us now turn to Dean's drawing showing the earliest 

 circulation on an egg of Chlamydoselachus found by him. 



Vitelline Circulation in the 39-mm. Embryo 



One of Dean's three drawings in color for the embryology of Chlamydoselachus 

 portravs this embryo and yolk (Figure 50, plate V). In this the artist has shown the 



