PAIRED FINS WITH THEIR NERVES, IN LEPIDOSIREN AND PROTOPTERUS. 623 



The cases of the constrictor pharyngis and of the glottis therefore present before 

 our eyes a partial recapitulation of the process which has resulted in the present position 

 of the posterior visceral arches ventral to trunk myotomes which really belong to a 

 region posterior to them. For the disappearance of the myotomic mesoderm properly 

 belonging to the branchial region has resulted in a forward movement of the anterior 

 trunk myotomes over the ventral portion of this region — which includes everything 

 innervated by the vagus — to partially take its place. 



II. The Occipital and Anterior Trunk Myotomes and Nerves. 



There are three occipital myotomes of Lepidosiren and Protopterus (x, y, z). Of 

 these, myotome x wholly disappears, y gives rise to the dorsal part of the constrictor 

 pharyngis, and also contributes to the hypoglossal musculature (coraco-hyoid), z con- 

 tributes to the coraco-hyoid, as does also myotome 1. Myotomes 2, 3, 4, and in 

 Protopterus 5, contribute to the musculature of the pectoral fin.* 



IIa. Cervical (Hypoglossal) and Brachial Plexuses and Occipital Nerves. 



Text figure 6 shows clearly enough the condition of these plexuses in a Lepidosiren 

 of stage 38. The cervical plexus is formed from the nerves y, z, and 1, the brachial 

 plexus from 1 and 2. As will be shown presently, the nerve fibres joining the 

 brachial plexus through Nl probably do not enter the fin. 



In younger Lepidosiren larvae there is an additional branch to the brachial plexus 

 from N3. This is shown in the figure as a dotted line (3 br). This branch runs under 

 the occipital rib, the trunk of Nl + N2 over it. The branch from N3 can be observed 

 to be atrophying in successive stages, and ultimately apparently disappears. 



In Protopterus the brachial plexus is much more extensive, reaching from Nl (or 

 probably, as in Lepidosiren, more strictly speaking from N2 — see p. 629) to No 

 inclusive. The constituents from NN 3, 4, and 5 are joined into a collector which runs 

 underneath the occipital rib like the branch from N3 in Lepidosiren. 



Henceforth the myotomes are spoken of as Mx, My, Mz, Ml, M2, etc., and the nerves as Nse, Ni/, etc. 



