172 MALL. [Vol. V. 



When the anterior and middle air sacs on the right side are 

 separated from the posterior sac on the same side (which is quite 

 easily done with the handle of the scalpel), a slit is shown which 

 extends to the oral and dorsal sides of the hepatic veins. Here 

 it communicates, by means of a round opening about one centi- 

 metre in diameter, with a large cavity lying on the median and 

 dorsal side of the proventriculus and extending to the spleen. 

 The cavity does not extend over the dorsal side of the gizzard. 

 In all respects, this corresponds with the right gastric diver- 

 ticulum and with the lesser peritoneal cavity in mammals. 



The relation of this embryonic omentum to the "pseudo- 

 epiploon " is as yet unknown. However, I think it probable 

 that the one is changed into the other, and that the "pseudo- 

 epiploon will prove to be the true epiploon, homologous with 

 the same in mammals. We must only imagine the embryonic 

 omentum attaching itself to the sides of the abdomen, followed 

 by a loss of the epiploonic peritoneal cavity. A subsequent 

 growth of the air cells back from the dorsal side of the stomach 

 will produce the condition found in the adult. 



Mammals. 



The earliest stage of mammals I have to examine is an em- 

 bryo of a dog, six millimetres long. 1 It represents about the 

 same stages as the chick of seventy hours already described. 

 In the chick the pericardial cavity communicated on all sides 

 with the pleuro-peritoneal cavity, while in this specimen the 

 communication takes place only on the aboral side of the sinus 

 reunieus. His pictures sections of human embryos, both earlier 

 and later than this stage; 2 and in all of them the communi- 

 cation of the pericardial cavity with the pleuro-peritoneal cavity 

 is only on the oral side of the ductus Cuvieri. The ccelom still 

 communicates freely with the false amnion. 



The stomach is still a straight, upright tube, communicating 

 freely behind with the umbilical vesicle. The liver is just aris- 

 ing as two lateral spouts of epithelial cells, each of which 

 encircles an omphalo-mesenteric vein. 



1 For description of this embryo, see my paper on " Branchial Clefts of the Dog," 

 Studies from the Biological Laboratory, Johns Hopkins University, Vol. IV. 



2 Anatomie menschlich. Embryonen, Atlas. Earlier stages, PI. XI, BB and LR; 

 later stages, PI. XII, R; II, B; IV, A. 



