236 CIRCULATION OF THE YOLK SACK. 



In fig. 2 tlie circulation was greatly advanced\ The 

 blastoderm has now nearly completely enveloped the yolk, 

 and there remains only a small circular space {yh) not en- 

 closed by it. The arterial trunk is present as before, and 

 divides in front of the embryo into two branches which turn 

 backwards and nearly form a complete ring round the embryo. 

 In general appearance it resembles the sinus terminalis of 

 the area vasculosa of the Bird, but in reality bears quite a 

 different relation to the circulation. It gives off branches only 

 on its inner side. 



A venous system of returning vessels is now fully de- 

 veloped, and its relations are very remarkable. There is a 

 main venous ring round the thickened edge of the blastoderm, 

 which is connected with the embryo by a single stem which 

 runs along the seam where the edges of the blastoderm have 

 coalesced. Since the venous trunks are only developed behind 

 the embryo, it is only the posterior part of the arterial ring 

 which gives off branches. 



The succeeding stage, fig. 8, is also one of considerable 

 interest. The arterial ring has greatly extended, and now 

 embraces nearly half the yolk, and sends off trunks on its 

 inner side along its whole circumference. 



More important changes have taken place in the venous 

 system. The blastoderm has now completely enveloped the 

 yolk, and as a result of this, the venous ring no longer exists, 

 but at the point where it vanished there may be observed a 

 number of smaller veins diverging in a brush-like fashion from 

 the termination of the unpaired trunk which originally con- 

 nected the venous ring with the heart. This point is indicated 

 in the figure by the letter y. The brush-like divergence of the 

 veins is a still more marked feature in a blastoderm of a suc- 

 ceeding stage (fig. 4). 



The circulation in the succeeding stage (fig. 4) (projected 

 in my figure) only differs in details from that of the previous 

 stage. The arterial ring has become much larger, and the 

 portion of the yolk not embraced (x) by it is quite small. 



1 My figure may be compared with that of Lcydig, Hochen unci Haie, Plate 

 III. fig. 6. Leydig calls the arterial ring the sinus terminalis and appears to 

 regard it as venous, but bis description is so sliort that this point is not quite 

 clear. 



