PAIRED FINS WITH THEIR NERVES, IN LEPIDOSIREN AND PROTOPTERUS. 631 



IK. Development of the Pelvic Fin. 



The following account applies to Lepidosiren. 



The pelvic fin makes its appearance at a slightly later stage than the pectoral. 

 The latter is first indicated in stage 30, the former in stage 31. Here there is a great 

 proliferation of cells from the somatopleural mesoderm taking place in the region of the 

 future fin rudiment. The ectoderm has also begun to be thickened in the same place, 

 viz. opposite M56. The limits of the ectodermal thickening are hard to determine 

 exactly, but it certainly does not extend appreciably beyond M56 on either side. As 

 regards the myotomes, all of these behind the first few are giving off mesenchyme cells 

 from the ventral processes (ventral connective tissue of Maurer), and this fact makes it 

 impossible at this early stage to determine whether they have begun to give ofT mesoderm 

 cells to the fin or not. At any rate, if they have, this process has not proceeded so far 

 as the modification in the somatopleural mesoderm and ectoderm. 



In stage 31 + the fin rudiment is well established ; there is now a distinct projection 

 from the surface (Plate fig. 7). All these sources of the fin material — somatopleural, 

 myotomic, and ectodermal — are in great activity. The ectodermal thickening, due to 

 a proliferation of nuclei, is considerable. It extends in the embryo examined from the 

 middle of M49 to the hind end of M52 (this corresponds to MM53-56 of the embryo 

 mentioned in the previous stage, the cloaca in the latter being opposite M58 instead of 

 M54 as in the former. This point will be returned to later). This is the greatest 

 extent of ectodermal rudiment, there thus being a very long interpterygial zone without 

 any ectodermal modification. 



It is impossible to put an exact limit to the number of myotomes taking part in the 

 fin rudiment. As in the previous stage, all the myotomes behind the first few are giving 

 off connective tissue cells from their ventral processes, but the process is enormously 

 exaggerated in the case of the myotomes just dorsal to the ectodermal thickening, 

 diminishing rapidly from the myotomes behind and gradually from the ones in front. 

 From the examination of sagittal sections of this stage, however, there appear to be 

 8 myotomes taking part in providing the fin rudiment with mesoderm cells. This 

 corresponds with the maximum number of nerves found taking part in the pelvic plexus. 

 This plexus in a larva of stage 34 was composed of NN45-53, and in stage 38 of 47-53. 

 In both cases there was also a doubtful branch from N54. 



So far as it goes, the Protopterus material shows the same features as Lepidosiren. 



The accompanying table gives the position of the cloaca and of the pelvic fin in 

 larvae of various stages. The position of the limb is defined by the attachment of its 

 post-axial border, as, owing to the backward slope of the limb, this point is sharply 

 marked. In the first three the position of the posterior limit of the ectoderm rudiment 

 is given. 



