PAIRED FINS WITH THEIR NERVES, IN LEP1D0SIREN AND PROTOPTERUS. 635 



or mesoderm gives the first visible evidence of the presence of the fin rudiment. In 

 Ceratodus (Semon) the initial visible changes take place in both simultaneously, in both 

 fins. In Lepiclosiren I have described the changes as taking place approximately 

 simultaneously in somatopleural mesoderm, myotomes, and ectoderm. If, however, 

 either of the three starts first (in the pelvic fin), that one is probably the somatopleural 

 mesoderm. Braus (p. 504), after reviewing the literature, comes to the conclusion that 

 such variations exist in this respect in Elasmobranchs that no phylogenetic importance 

 can be attached to it. As a rule, the first visible changes take place in the mesodermal 

 structures, though the ectoderm has been described as being the first to exhibit the 

 change in some forms (Balfour). Thus, so far as the actual visible course of develop- 

 ment of the fins is concerned (especially the pelvic), the balance of evidence seems to' 

 be against the ectodermal portion of the rudiment acting as a stimulus to the meso- 

 dermal. The conditions in Lepiclosiren, however, suggest that the ectoderm begins to 

 exert its influence before visible changes take place in it. Here all the myotomes 

 muscularising the pelvic fin are dorsal to or in front of the fin rudiment (ectodermal 

 part), the ventral processes of the more anterior ones sloping strongly backward. I 

 should interpret this, on the view here set forward, as due to the fact that the area of 

 ectoderm which at stage 3 1 shows a proliferation of nuclei, the first visible ectodermal 

 part of the fin rudiment, has been travelling backwards from a more anterior position, 

 and by the time it has reached M45 (the most anterior myotome taking part in the fin 

 muscularisation), has begun to take on the character of fin rudiment, though this has 

 not yet resulted in a visible structural change. Still travelling back, it pulls the ventral 

 processes of the myotomes after it, and by stage 31, when structural changes first 

 appear, it is under M54. 



The fact that the pelvic fin appears later than the pectoral is in accordance with 

 this view, though it cannot be said to offer positive evidence in favour of it, the differ- 

 ence in time of the appearance of the two limbs being sufficiently accounted for by the 

 general lagging behind of the development of the posterior region of the body. 



IV. Summary. 



The head mesoderm in Lepiclosiren and Protopterus never shows a segmentation 

 corresponding to the trunk segmentation. 



The eye muscles are derived from a continuous, extended source, probably corre- 

 sponding to the first three somites of van Wijhe. 



The mesoderm included in the first four visceral arches is, at the time of their 

 formation by the meeting of the gill outgrowths with the ectoderm, attached to 

 unsegmented somatic head mesoderm. That included in the fifth, sixth, and seventh 

 TRANS. ROY. SOC. EDIN., VOL. XLV. PART TIL (NO. 23). 90 



