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



arches is ventral, but not attached, to the occipital myotomes, and does not therefore 

 necessarily belong to them. The development of the pericardium in Lepidosiren and 

 Protopterus, etc. shows by analogy that even when the mesoderm cut oft' by the 

 posterior visceral arches is attached to the occipital myotomes, it is not necessary to 

 believe that they belong genetically to the same somites. Probably the branchial 

 region was formerly entirely in front of the occipital region which now overlaps it. 

 The fourth pro-otic segment of van Wijhe probably represents a large number of fused 

 myotomes to which all except the first two visceral arches belonged. 



The coelome and pericardium are connected at one stage by paired dorsal pericardio- 

 peritoneal ducts, and by paired ventro-lateral passages between the liver and body wall. 

 The inner walls of the pericardio-peritoneal ducts give rise to the ventral and lateral 

 parts of the constrictor pharyngis. The dorsal part of this muscle is of somatic origin, 

 being derived from myotome y. The muscle appears to undergo a change of function 

 during ontogeny, from a simple constrictor to a separate compressor and dilatator. 



The region of the mesodermal investment of the alimentary canal innervated by 

 the vagus, both by its muscular and sensory fibres, actually extends to successively 

 more and more posteriorly situated myotomes during development, owing to differential 

 growths of the two structures. This gives us a partial recapitulation of the process 

 which has resulted in the present position of the posterior visceral arches under trunk 

 myotomes which belong genetically to a region behind them. The incentive to this 

 differential growth of trunk myotomes over palingenetic head structures was the dis- 

 appearance of the myotomic mesoderm of all that region innervated by the vagus. 



There are three occipital myotomes, x, y, z. Of these, x vanishes entirely, y forms 

 the dorsal part of the constrictor pharyngis, and contributes to the hypoglossal muscu- 

 lature. Myotome z contributes to this musculature, as does also Ml. MM 2, 3, 4 in 

 Lepidosiren, MM 2, 3, 4, 5 in Protopterus, muscularise the pectoral fin. 



The occipital myotomes do not migrate relatively to the central nervous system. 



The cervical (hypoglossal) plexus consists of NNy, z, 1, and the brachial of NN?1, 

 2 (adult Lepidosiren), ?1, 2, 3 (young Lepidosiren larvae), ?1, 2, 3, 4, 5 {Protopterus). 



The nerve x has not been found in any specimen. 



Ny and Nz have ventral roots only. 



Nl has generally a dorsal ganglion in Protopterus, generally not in Lepidosiren. 

 In the latter genus especially this vestigial structure is subject to a high degree of 

 variation. 



The anterior pronephrostome seems to be quite constantly in Lepidosiren, and 

 nearly constantly in Protopterus, under Ml. 



The relations of the occipital myomeres, neuromeres, and scleromeres show that the 

 single occipital arch in all young Lepidosirens and most # young Protopterus indicates 

 a truly protometameric skull, and not one that has undergone reduction from an 

 auximetameric condition. 



* See Trans. Roy. Soc. Edin., vol. xlv. 



