64 EDWARD MCCRADY, JR. 



arose, run together in the mesenchyme under the midbrain 

 and grow back between the otic placode and the medullary 

 plate. They pass the second branchial pouch (which will be 

 described below) on its medial side (later dorsal) and end 

 blindly except for scattered capillary communications with 

 the dorsal aorta. These vessels constitute the anlagen of 

 the primary head vein. 



At the same time a similar plexus appears in the parietal 

 mesenchyme dorsal to the point where the vitelline vein enters 

 the body. From this plexus one channel eventually becomes 

 enlarged. It lies (fig. 20, I) close against the parietal peri- 

 cardium, which in turn is in contact with the visceral peri- 

 cardium at this point, but no communication is attained until 

 late stage 25. From this level it grows caudad until its pos- 

 terior tip reaches the limb ridge (fig. 20, F). This vein, the 

 umbilical, is thus in early stages exclusively a vessel of the 

 anterior limb and the lateral body wall, and in the opossum 

 this primary association is preserved until a relatively late 

 stage of development (stage 30, see p. 165). 



In the latest specimens of stage 24 (e.g., 17136, 12 somites) 

 a continuous border vessel is formed around the area vascu- 

 losa the sinus terminalis. It has innumerable communica- 

 tions with the yolk sac vessels and through them eventually 

 with the vitelline and aortic plexuses from the embryo. 



The first sensory anlagen. In stage 23 no sensory anlagen 

 were found, though it should be remembered that my material 

 for that stage is inadequate. In the earliest specimen of 

 stage 24 which I have (17153) both the optic cups and the otic 

 placodes are present, so that I cannot establish any priority 

 between these two. 



As far as my evidence goes the optic cups are paired and 

 lateral from the beginning (fig. 20, totomount, A, I, K, L). 

 At first they are exceedingly difficult to recognize in toto- 

 mounts unless the lighting is carefully adjusted. But they 

 show very clearly in the photograph (fig. 19, A) and I do 

 not doubt that if Selenka's light had been satisfactory he 

 would have found them in the specimen he figured in his 



