DEVELOPMENT OF THE HEART IN MARSUPIALS. 



461 



Laterally to the somitic mesoderm and the medullary plate is 

 a zone appearing clear in svirface view, in which the mesoderm 

 forms a thin sheet. Outside this again is an opaque zone of 

 lateral mesoderm (Z.il/.), completely surrounding the embryo. 

 This is bounded peripherally by a clear zone which separates it 

 from the area vasculosa (Av.). The entoderm forms a thin 

 continuous layer, while a small incipient head-fold definitely 

 marks the anterior margin of the brain-plate and involves also 

 the protochordal plate lying in the middle line immediately 

 below it. 



In the lateral mesoderm on each side a horizontal cleft has 

 appeared, separating the mesoderm into a dorsal somatic and a 

 ventral splanchnic layer. These clefts, commencing on both 

 sides of the embryo and extending forwards, constitute the 

 pleuro-pericardial canals, the form of which can readily be seen 

 in PI. I. fig. 1 {P. 23.0. ) The canals foxnn a horseshoe the median 



Text-figure 2. 



SdAf^-^ 



Ferameles obesida (1 Z, B). Transverse section in plane A, Ai, 

 (See text-fig. 1.) 



!Ent. Entoderm. M.F. MecIuUarj' plate. F.p.C. Pleuro-pericardial canal. 

 S.M. Splanchnic mesoderm. So.M. Somatic mesoderm. 



:anterior portion of which lies underneath the anterior margin of 

 the brain-plate (M.M.P.), while the lateral limbs extend back 

 into the somitic region. In the anterior region, the dorsi-ventral 

 extent of the pleuro-pericardial canal is very small (text-fig. 2, 

 P.p.C), and in fact the continuity of the canal is actually inter- 

 rupted on the right side of the embryo (PI. I. fig. 1). 



The cleft increases markedly in size in the region of the 

 trigeminal neural crest proliferation (text-fig. 1, N.C.P. {T.G.)) 

 and attains its greatest size in the hind-brain region (text-fig. 3 A 

 & B). Behind this it becomes reduced in size until, opposite the 

 posterior somites, the coelomic cavity is represented by irregular 

 clefts in the lateral mesoderm. 



The pleuro-pericardial canals throughout their extent have a 

 thin somatic and a slightly thicker splanchnic wall (see text-figs. 2, 

 3 A & B, So.M., S. 21.), which in the anterior region is in close 



32* 



