GILL-FILAMENTS IN SAUROPSIDA 215 



As far as one can judge from the meagre description of the 

 region in mammals, it seems probable that the pectoral wall is 

 vascularized by a different method than that which obtains in 

 the chick. Evan's figure of a fifteen millimeter pig embryo sug- 

 gests that the pectoral wall is invaded by the same capillary net 

 which grows down from the cervical region in the chick, but in 

 this animal continues until it reaches the umbilicus. Kolliker's 

 figure of a cow embryo of about the same stage suggests that this 

 area is supplied by the capillary net which grows in from the 

 marginal veins. Mall describes the condition in man as follows : 



I have in my collection a well-preserved human embryo (no. LXXVI) 

 f22 days), in which the membrana reimiens is filled with a plexus of 

 veins much like that in the cow's embryo .... The ventral 

 wall of the heart near the liver contains no vessels, while the membrana 

 reuniens covering the upper end of the heart is filled with a plexus of 

 vessels which communicate with the capillaries of the mandibular arch. 



The study of the blood vessels thus confirms the testimony of 

 the superficial markings, that there is a considerable shifting of 

 body-wall tissues from the sides of the embryo toward the mid- 

 ventral line, — this in advance of the later invasion of pectoral 

 muscles, nerves, dermal and skeletal parts which enter into the 

 formation of the definitive pectoral wall. Whether there is any- 

 thing of this preliminary movement in other amniotes is difficult 

 to determine from the evidence at hand. But certainly in chicks 

 of the sixth and seventh days it is possible to demonstrate a 

 medial displacement of tributaries of the umbilical veins and a 

 shifting of an internal and external set of ridges. Although I have 

 been unable to find in the literature any record of such ridges or 

 grooves as have just been described, there is some evidence that 

 the older embryologists who discussed this region in birds and 

 reptiles saw something of this early shifting of tissues. Thus 

 Rathke, the first to maintain that the muscle and skeletal ele- 

 ments of the breast wall did not arise in situ, believed that these 

 structures grew in from the sides of the embryo in such a way 

 as to push ahead of them the thin somatic wall which originally 

 covered the thoracic and abdominal viscera, eventually causing 

 the membrana reuniens to disappear completely (allmiilig ver- 



