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



IIb. Embryonic Condition of the Occipital and Anterior Trunk 



Nerves and Myotomes. 



In no larva of either genus have I found the nerve a;.* The nerves y and z are 

 always present ; N?/ is very thin. Both have ventral roots only. Sometimes Nl has 

 a dorsal ganglion, sometimes not. This is more often present in Protopteras than in 

 Lepidosiren. In both genera there is a great deal of variation in regard to this point, 

 irrespective of the stage of development. In Lepidosiren the ganglion 

 is present on both sides in 2 specimens — stages 29 + and 34. 



,, one side only in 6 ,, — ,, 30, 30, 34, -35, circ. 35, 38. 



is absent altogether in 12 „ — „ 31, circ. 31, 31 + , 32, 32, 32 + , 



32 + , -35, -35, 36, 36 + , 38. 



a.o.p. 



V.--nas. caps. 



mand. 



cor. hy. 



Fig. 6. — Cervical and brachial plexuses in Lepidosiren, from a reconstruction (by Graham Kerr's method), of a larva of stage 

 38. The chondrocranium at this stage is shown, a.o.p., ant-orbital process ; aud.caps., auditory capsule ; br.n., brachial 

 nerve ; cor.hy., coraco-hyoid muscle ; c.ph.7i., nerve supplying constrictor pharyngis (dorsal part) ; hy., hyoid ; hypog.n., 

 hypoglossal nerve; mand., mandible; nas.caps., nasal capsule; occ.arch., occipital arch; occ.rib., occipital rib ; p.g., 

 pectoral girdle ; quad., quadrate ; My, Mz, Ml, etc., the myotomes ; y, z, 1, etc., the nerves ; 3 br., branch from spinal 

 nerve 3 to brachial plexus, added from a younger specimen. It can no longer be traced at this stage. 



In Protopterus I found the ganglion present on both sides in 3 larvse, of stages 30, 

 32, and 5"5 cms. on one side only in 2 larvae, both of stage circ. 36, absent in one of 

 stage circ. 36. 



It is thus seen that the presence or absence of the ganglion is by no means alto- 

 gether determined by the age of the specimen, but that we are dealing with a highly 

 variable vestigial structure. 



The succeeding nerves always have both dorsal and ventral roots. 



Throughout development the occipital myotomes remain in the same position 

 relative to the central nervous system, and, after its formation, to the occipital arch. 



* I have not, however, made an exhaustive search for this nerve, not considering that the labour involved in 

 proving its absence in a long series of embryos would be repaid by the value of the knowledge gained. Pinkus and 

 Furbringer found Nx in specimens of Protopterus. 



