620 MR W. E. AGAR ON DEVELOPMENT OF ANTERIOR MESODERM, AND 



mesoderm is unsplit, while in the head the somatopleural and splanchnopleural 

 mesoderm have separated in the anterior median portion of the lateral plate to form the 

 rudiment of the pericardium. This cavity extends also for a short distance backwards 

 into the still paired portion of the lateral plate on each side. 



By stage 30 the lateral plates have not yet met below the yolk sac in the trunk. 

 They have split in the region of the first few trunk myotomes. The cavity so formed 

 (coelome) is continued forwards in the dorsal part of the lateral plate in front of this 

 region, right up into the pericardium (pericardio-peritoneal ducts — Hochstetter). At 

 present these narrow canals form the only communication between pericardium and 

 general coelome.* 



At a slightly later stage (31 + ) the ventral parts of the lateral plates between 

 pericardium and the rest of the coelome have split, leaving an intermediate part which 

 never splits. The coelome and pericardium are therefore now connected — 



1. By paired dorsal pericardio-peritoneal ducts. These connect the pericardium with 



the pronephrocoeles, which are now nearly cut off from the rest of the coelome. 



2. By wide paired, ventro-lateral passages, running between the liver and body wall. 

 Both these connections become obliterated about stage 34 or 35. 



At stage 30 the pericardio-peritoneal duct on each side lies, posteriorly, on the 

 dorso-lateral border of the yolk sac ; but further forward, where the oesophagus is being 

 constricted off from the underlying yolk sac, it lies in the angle between the two. 

 Further forward still, where the pharynx is completely folded off, the ducts lie on its 

 ventral side. It lies altogether ventral to the branchial ridge. 



The inner walls of these ducts, which are applied to the solid mass of endoderm 

 representing the developing pharynx, are seen at this stage (30) to be budding off 

 mesenchymatous cells between themselves and the pharynx (Plate fig. 1). Over the 

 rest of the wall of the duct the nuclei are distributed very sparsely and have their long 

 axes parallel to the direction of the wall, while on the inner wall the nuclei are thickly 

 packed, have their long axes at right angles to the wall, and are several layers thick. 

 The nuclei of the outer layers are less closely packed, as if moving away from their 

 place of origin. 



By stage circ. 31 these cells have accumulated enormously, and form a thick layer 

 investing the pharynx ventrally and laterally. 



In stage 31+ we find muscle fibres developing in this mass of cells, forming the 

 ventral and lateral parts of the constrictor pharyngis of Wiedersheim. 



The ramus muscularis of the vagus can be definitely traced into this muscle from 

 now onwards. 



At present the pharynx or oesophagus is free from muscle dorsally, but at this 

 stage (31 +) a ventral outgrowth of cells from the occipital myotome y (possibly, but 

 not probably, the outgrowth extends on to x and z also) is taking place. This is the 



* I here use the term coelome in its widest sense, without prejudice to the question as to whether the pro- 

 nephrocoeles belong to segmented or unsegmenled mesoderm. 



