48 ARTHUR DENDY. 



stage. The appearance of tlie living embryo as seen from 

 above is represented in fig. 93. The embryo beneath the thin 

 transparent serous envelope lies lengthwise in a well-defined, 

 oblong, clear area of the yolk-sac, corresponding more or less 

 closely in extent to the original area pellucida. Outside this 

 clear area the yolk-sac is very heavily laden with yolk, which 

 disappears abruptly at its margin. 



The sinus terminalis {S.T.) now lies at a considerable 

 distance outside the clear area of the yolk-sac, enclosing an 

 oval space about 10 mm. long by 8*8 mm. broad. Anteriorly 

 its two halves meet to form a large vein which runs backwards 

 and somewhat to the left, turning inwards at the left-hand 

 end of the fold (TV. L.) along which the yolk-sac is invaginated 

 around the anterior end of the embryo to form the outer layer 

 of the [)ro-amnion. In this vessel the blood flowed in a steady 

 stream towards the heart, which was beating about thirty-eight 

 times per minute. The larger vitelline vessels on the right 

 and left sides can be traced inwards as far as the edge of the 

 clear area of the yolk-sac, where they become lost to view 

 from above, dipping down to follow the splanchnopleure in- 

 wards along the floor of the pleuro-periloneal cavity. 



Fig. 92 represents the same embryo seen from below as 

 an opaque object after hardening. The anterior half of the 

 embryo, up to a point just behind the fore-limbs, is pushed 

 into the yolk-sac, and in life hangs freely down into the yolk, 

 enveloped in a thin transparent membrane (the pro-amnion), 

 composed of the true amnion inside, and an outer layer which 

 really belongs to the yolk-sac but which is not vascular. (In 

 hardened specimens the head is turned either to the right or 

 to the left, according to the way in which the embryo 

 happened to lie in the hardening fluid.) 



The following features in the embryo itself may be regarded 

 as characteristic of the stage : 



The ventral flexure of the body in the region of the 

 shoulders is more strongly marked, so that the head almost 

 touches the allantois (AIL), which has increased considerably 

 in size. 



