120 EDWARD MCCRADY, JR. 



splanchnic mesoderm. The end of the hollow stalk is capped 

 by a sheet of somatopleure the serosal chorion. In stage 29 

 the two splanchnopleuric faces carry many small vitelline 

 vessels (mostly arterioles from the dorsal aortae), but in 

 later stages only the three large vessels survive. 



Though I have written this description of the foetal mem- 

 branes from my own observations, most of the facts had been 

 worked out before, and accordingly a few words about the 

 literature are necessary at this point. 



Osborn's three papers on the foetal membranes of the 

 opossum (1883 a, 1883 b, 1887) are so full of errors that it 

 seems impossible to understand what sort of observations 

 they could have been based upon. Even the third paper, 

 which purported to correct the errors recognized in the first 

 two, introduced an almost equally rich crop of new ones. It 

 would be confusing and unnecessary to dwell on all these 

 mistakes, but one of them which persisted through all three 

 of his publications seems to have been borrowed from Chap- 

 man's paper on the foetal kangaroo (1881). This author, 

 who examined grossly a single 14-day Macropus foetus, in- 

 correctly described the sinus terminalis as marking the 

 boundary between the chorion and the umbilical vesicle, not 

 realizing that the umbilical vesicle or yolk sac forms the 

 lining of the entire vesicle with the exception of the very small 

 serosal spot mentioned above. While this error is not in- 

 comprehensible when based only upon gross observation, it 

 is difficult to understand how Osborn could have failed to de- 

 tect it in his microscopical slides. At any rate it was not cor- 

 rected until the second half of Selenka 's paper was published. 

 Selenka's account of the development, anatomy, and signifi- 

 cance of all the foetal membranes is remarkably accurate and 

 complete. 



In fact, I have come across only one error, and that a minor 

 one, in his lengthy account. He described the membrane 

 which forms the anterior wall of the body stalk between the 

 two omphalomesenteric veins as including a double layer of 

 ectoderm between its endodermal and mesodermal constitu- 

 ents (loc. cit., p. 133). This double layer of ectoderm does 



