PAIRED FINS WITH THEIR NERVES, IN LEPIDOSIREN AND PROTOPTERUS. 627 



the hypoglossal musculature (coraco-hyoid), and the three [Lepidosiren) or four 

 (Protopterus) behind provide mesoderm for the pectoral fin. 



Development of the Coraco-hyoid Muscle. 



The disposition of the ventral processes which go to form the coraco-hyoid in 

 Protopterus is seen in text figure 7. The figure would do as well for Lepidosiren as 

 regards this point. 



At stage 30 these three ventral processes have separated from their myotomes and 

 travelled downwards and forwards. They are so completely fused together that no 

 trace of a division between the contributions of the different myotomes could be found. 

 As development proceeds, the compact mass of mesoderm so formed on each side 

 gradually grows forwards along each side of the pericardium, finally to meet each other 

 in front of and ventral to it, and become inserted in front into the rudiment of the 

 hyoid arch and behind into the pectoral girdle. Thus the strong coraco-hyoid muscle 

 is formed. On its differentiation into muscular tissue, the rudiment becomes divided 

 up, by a connective tissue intersection, into two segments (Ml and MMz + y) 

 (text figure 6). Furbringer finds that in the (adult) specimens of Protopterus 

 examined by him the anterior of these two segments was partially subdivided into 

 two, which we now see to be Mz and My. I could not find this subdivision in any 

 of my larval Protopterus, nor in larval or adult Lepidosirens. However, in Protopterus 

 My contributes much less substance to the muscle rudiment than Mz or M 1 , and in 

 Lepidosiren still less. This is also shown by the very small bundle of nerve fibres 

 contributed by Ny to the cervical plexus. 



Development or the Pectoral Fin. 



The mesoderm of the pectoral fin is supplied by the ventral processes of MM 2, 3, and 

 4 in Lepidosiren, and 2, 3, 4, and 5 in Protopterus. The general disposition of these 

 ventral processes, and their relations to the pronephros, etc. in Protopterus, is shown in 

 figure 7. This text figure would apply equally well to Lepidosiren if the ventral process 

 of M4 were made like that of M5, and that of M5 like M6, etc. 



We will take our detailed account from Lepidosiren. As already mentioned, the 

 mass of the pronephros causes the ventral process of M2 to be displaced backwards, and 

 it runs out through a narrow space opposite myoseptum M2-M3, left between the 

 posterior pronephrostome, ventral process M3, and ectoderm. Presumably in con- 

 sequence of its restricted space, it very early loses its epithelioid character, and 

 breaking up into mesenchyme, looses itself from its myotome, and slips ventro-laterally 

 TRANS. ROY. SOC. EDIN., VOL. XLV. PART III. (NO. 23). 89 



