478 A. M. MILLER 



vascular channels and islands in the mesenchymal tissue dorsal 

 to the portions of the precardinal and postcardinal veins near 

 their confluence to form the duct of Cuvier. All these channels 

 and islands are obviously venous in character, many of them 

 filled with nucleated red blood cells. 



Compared with the mammalian embryo, the conditions in 

 the chick at this stage correspond with those in a cat embryo of 

 6.2 mm. (Huntington and McClure, figs. 27, 28, 29 and 30). 

 In the chick, however, the vascular islands and channels belong- 

 ing to the lateral group have not coalesced to such a degree as 

 the corresponding veno-lymphatics in the cat. In the cat the 

 precardinal receives several dorsal tributaries while in the chick, 

 at this stage at least, the dorsal somatic veins proper of this 

 region open into the postcardinal. 



Chick ejnhryo of five days and ten hours, 13.5 mm. Reconstruc- 

 tion, right side. Fig. 3. Although this embryo is younger than 

 the preceding, it is farther advanced in development, due probably 

 to different conditions in incubation. 



The precardinal vein (1) has become elongated both relatively 

 and absolutely, and pursues a moderately curved course from the 

 cephalic arch to the duct of Cuvier (5). The proximal portion 

 of the postcardinal (2) is relatively smaller than in the preceding 

 stage, and appears rather as a continuation of the primitive ulnar 

 vein than as a part of the original postcardinal. There is no 

 promontory in the cardinal line such as, in the previous stage, 

 was produced by the sharp bend in the proximal end of the post- 

 cardinal (compare figs. 1 and 3). 



There is in this stage a still more distinct separation of the 

 vascular channels and islands (5) dorsal to the cardinal veins 

 {1, 2) into a medial and a lateral group than in the stage consid- 

 ered before. Here again the medial group obviously represents 

 the dorsal somatic (cervical intersegmental) veins proper (4), 

 while the lateral group [5) exhibits a stronger tendency toward a 

 plexiform arrangement. 



As in the preceding stage, none of the dorsal somatic veins {Jj) 

 open into the precardinal {!). Along the dorso-lateral aspect of 

 the aorta, however, as far back as the level of the duct of Cuvier 



