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



large yolk sac, such as is found in the Dipnoi ? In Lepidosiren and Protopterus it 

 seems a mechanical impossibility for Ml to participate in the muscularisation of the 

 pectoral fin, or for M2 to do so in the coraco-hyoid. Here at any rate we have a con- 

 dition which almost precludes the muscularisation of these two regions from any common 

 myotome — a condition which would lead to the stage of things found in the terrestrial 

 vertebrates. 



A slight variation in the position of the anterior pronephrostome will of course 

 have to be accompanied by a corresponding shifting of the division between the 

 two plexuses. 



So far as Lepidosiren and Protopterus are concerned, there is strong evidence that the 

 pectoral fin has recently undergone a forward migration, and Braus (1898) comes to the 

 same conclusion for Ceratodus. We find, in the two former genera, that the fin is 

 situated in front of the myotomes participating in its muscularisation, and the evidence 

 is conclusive that posterior myotomes are successively ceasing to take part in it. In 

 Protopterus the first of the brachial plexus nerves, N2 (not counting the contribution 

 from Nl), is the largest, and the size of the branches from the other nerves diminishes 

 as one passes backwards. Also M2 and M3 contribute the bulk of the mesoderm. In 

 Lepidosiren the reduction has gone further, for the branches from N5 and N4 have 

 gone completely, and that from N3 vanishes during development. We are thus left 

 with the remarkable result that the pectoral fin in Lepidosiren is probably innervated 

 by one spinal nerve only (N2). 



A summary of the derivations of the anterior myotomes in the Dipnoi is given in 

 the following table. Necturus is added for comparison. 





Position of 



Anterior 

 Pronephro- 

 stome. 



Cor 



aco-hyoid. 







Pectoral Fin. 





Myotomes 

 muscularising. 



Nerves innervating. 



Myotomes 

 muscularising. 



Nerves innervating. 



Ceratodus 



Ml. (S) 



y, z. (S) 



x, y, z. 



y, z- 



(F) 



(F) 



12 3 



j., a, it. 



(S) 



a,b, c(3). (S) 



z, a, b, 3, 4-11. (B) 



a, b, 3, 4, 5. (F) 



a, b, 3, 4, 5, 6. (F) 



Protopterus 



Ml. (A) 



y, 2 ,1. (A) 



y, «, 1. 



x, y, z. 



(A) 

 (F) 

 (F) 



2, 3, 4, 5. 



(A) 



11, 2, 3, 4, 5. (A) 

 a, b, 3. (F) 

 a, b, 3, 4. (F) 



Lepidosiren 



Ml. (A) 



y,z,l. (A) 



y, z, l. 



(A) 



2,3,U. 



(A) 



11, 2, (and 3 in embryo). (A) 



Necturus 



M2. (P) 



*,1,2. (P) 



1, 2, 3. 

 2,3. 



(F) 



(F) 







Authorities: — A, the Author; B, Braus; F, Furbringer; P, Miss Platt; S, Semon. 



