DEVELOPMENT OF THE HEART IN MARSUPIALS. 



467 



Shortly behind this, the paraxial mesoderm is differentiated 

 into somites. 



The maxillo-mandibular process (text-fig. %,Mm. arch) forms a 

 dense mass of mesoderm not distinctly marked out into maxillary 

 and mandibular portions. It lies laterally to the gut on the 

 outer side of the lateral and dorso-and ventro-lateral walls of the 

 gut, and extends antero-posteriorly from almost the anterior end 

 of the gut back to the first visceral pouch, which is situated level 

 with the anterior intestinal portal. The entoderm of the first 

 visceral pouch reaches the ectoderm, but the closing membrane 

 is not perforated. 



Text-figure 6. 



Perameles nasuta. Dorsal view of embr^'o A. 



B, Bi indicates the level of the section represented in text-fig. 7. 



^.«. Auditorj' vesicle. F.N. Facial neuromere. S.arcJi, Hyoid arch. Mm. arch, 

 Maxillo-mandibular arch. M.P. Medullary plate. S. Somite. 



The hyoid arch is a mass of mesoderm only slightly smaller in 

 surface view than the maxillo-mandibular arch, but situated 

 entirely dorsal to the gut. Behind it is the second visceral 

 pouch, which is small and does not reach the ectoderm 

 (PI. I. fig. 4, r.P. 2). 



The form of the pericardium and its relations to the gut, 



