460 MR. G. W. BUTLER ON THE SUBDIVISION OF [NoV. 19, 



named the " recessus superior sacci omenti " in Mammalia, whose 

 homologues in the Bird are much more conspicuous, that it occurred 

 to me that these recesses and their bounding walls might serve as 

 landmarks, and enable us more definitely to contrast the diaphrag- 

 matic structures in the two classes (c/, infra). 



III. (c). On the Development of the Pulmoliepatic Ligaments and 



Recesses. 



In this counexion it will be convenient to consider the develop- 

 mental changes in the chick classed under head (D) on p. 456. 



Referring to figures 1-4 (of which I and 2 are transverse sections 

 of a 6th-day chick, 3 of an embryo of Lacerta vivipara, 4 of an 

 embryo mammal *), we see running down the centre of the sections 

 the median vertical sheet of tissue (?«) which in all the types under 

 consideration forms a complete vertical septum in the thoracic and 

 anterior abdominal regions. On either side of it are set the lungs 

 and the two halves of the liver, while the alimentary canal runs in 

 its midst. 



It will further be noted that in all three cases the right liver-lobe 

 is attached to the lung of its own side by a vertical ligament 

 (a), which closes on the outside a space (2). The latter is blind 

 anteriorly and opens into the general peritoneal cavity behind. 



In fig. 1, which is a section anterior to fig. 2, we see that in the 

 chick there is, on the left side also, a corresponding ligament (a) 

 and included recess (2'), while in the case of the Mammal and (in 

 this instance) of the Lizard there is no such ligament traceable on 

 the left side, and therefore no recess, properly speaking, though 2' in 

 figs. 3 and 4 marks where it should be. Now the recess on the 

 right side (2) is the "recessus superior sacci omenti" of His, as 

 quoted by Kavu (9, p. 141), and the ligaments (a) are those which 

 in the Bird can be clearly traced developing into the pulmohepatic 

 li(/aments ; while the recesses, with the addition on the right side of 

 all that remains of the omental sac proper, develop into the 

 pulmohepatic recesses. 



According to Ravn {op. cit. figs. 15, 16, and text) this "recess" 

 on the right side is contnnious with the main omental sac as late as 

 about the 15th day in the Rabbit, but by the l/th day (p. 146) it 

 has become constricted off from the latter cavity and persists as a 

 separate closed peritoneal sac, which comes to wrap round the 

 oesophagus. 



By a reference to the Plates, the development, in the chick, of 

 these pultnoliepatic recesses Biaii ligaments m&y be followed. Figs. 

 1 and 2 are transverse sections of the 6th day (and the relations are 

 much the same even on the 4th day). Figs. 5, 6, 7 may be 

 called transverso-longitudinal sections (c/. fig. 25) of a 7th day 

 embryo. Of these tig. 6 shows the foramen of Winslow (y, W.) 

 leading from the main peritoneal cavity into the sac (2) of the right 

 side, part of which corresponds to the omental sac of Mammals. 



1 See also figs. 6-9, 11, 12, 14^18 of Eavn's paper (9). 



