GILL-FILAMENTS IN SAUROPSIDA 213 



median ridge has been observed in a human embryo of 45.0 mm. 

 (H. E. C, Ser. 2079) and in a dog embryo of 14.0 mm. (H. E. C, 

 Ser. 2052), but I have been unable to discover any clue to its 

 origin in these animals. It is possible that in mammals, only 

 the last stages of the process are visible. 



An examination of the inner surface of the pectoral wall shows 

 that an approximation of the two mesothehal ridges is also taking 

 place (fig. 1). In sectioned embryos each ridge was found to 

 contain one or two small veins although no blood vessels could 

 be detected in the area between the two ridges in their lower ex- 

 tent. This suggested a study of injected embryos which has 

 strikingly substantiated the shifting of tissues in the pectoral 

 wall. 



The displacement of veins in the pectoral wall. In a five-day 

 chick that portion of the body wall which covers the heart is 

 . entirely free from blood-vessels. On the margins of this roughly 

 triangular area lies a capillary network, continuous with that 

 which fills the rest of the vie7nbra7ia reuniens (Rathke's term for 

 the thin somatic wall which originally covers the abdominal and 

 thoracic viscera) . This appears to be growing into the wall over 

 the heart much as capillary-nets elsewhere invade non-vascular 

 regions. By the middle of the sixth day (injected embryo 5 

 days, 7 hours; 12.4 mm.) this network of the membrana reuniens 

 has resolved itself into a series of radial veins converging upon 

 the umbilicus from the myotomes. Those immediately adjacent 

 to the non-vascular triangle (that is, under the pectoral grooves) 

 converge upon the umbilical vein of their respective sides at 

 the point where it enters the septum transversum, on either side 

 of the apex of the heart. From now on, the non-vascular area 

 over the heart will become more and more circumscribed, not 

 by the ingrowth of new vessels (except in the uppermost part 

 which is congruous with the opaque area previously described, 

 where a capillary net grows down from the cervical region) but 

 by a shifting of marginal veins already formed. These swing in 

 on two pivotal points, the points referred to above, where the 

 umbilical vein of each side enters the septum transversum. 

 These radial venules are not straight lines but present a convex 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 23, NO. 1 



