PAIRED FINS WITH THEIR NERVES, IN LEPIDOSIREN AND PROTOPTERUS. 619' 



site to that of Neal or Koltzoff. We may regard the later extension of the branchial 

 region under the metotic myotomes 8-13 as a continuation of the process by which it 

 extended backwards under the anterior metotic myotomes (or these grew forward over 

 it). At first the branchial region was in the palingenetic part of the head. Then the 

 somatic portion of this began to atrophy, its place being taken by the anterior trunk 

 myotomes. This was doubtless helped by the disappearance of whole somites of the 

 branchial region, including their visceral arches and pouches, compensated by increase 

 in size of the individual gill clefts which were left, — a process which everyone agrees 

 has taken place. As the anterior trunk myotomes progressed forwards over the 

 branchial region, their sub-branchial processes got successively severed from their 

 dorsal parts. Owing to the shortening of the ontogenetic recapitulation of the 

 phylogeny, the branchial region reaches back as far as myotome 7 (metotic) before 

 these have begun to form their ventral outgrowths. Consequently the anterior ones do 

 not now form them at all, their place being taken by the outgrowths from the 

 myotomes directly behind the branchial region. As, however, the process of backward 

 extension into the trunk region is not finished by the time of the formation of the sub- 

 branchial processes, the continuation of it recapitulates more fully the phylogeny, being 

 accompanied by the successive severance of these processes from their myotomes. 



Note on the conception of the vagus as a collector for the splanchnic fibres of the 

 occipital nerves : — 



The only direct evidence I know of supposed to be in favour of this view is 

 the fact, made use of by Koltzoff, that in Petromyzon the dorsal anterior spinal 

 nerves anastomose with the ramus branchio-intestinalis. It is difficult to see, 

 however, how a peripheral anastomosis between these nerves can be a stage towards 

 the condition presumed by this theory for all higher vertebrates — that is, that the 

 splanchnic fibres of these spinal (now occipital) nerves leave the medulla through the 

 ramus branchio-intestinalis, instead of through the trunks of the nerves to which they 

 belong. Moreover, there seems to be no inducement for the splanchnic fibres of the 

 occipital nerves to shift their point of exit from the central nervous system forward 

 from the somatic fibres of the same nerves while the area of distribution of these 

 splanchnic fibres is at least not in front of that of the somatic ones. 



Ib. Lateral Plate Mesoderm in Head and Neck, and 

 Constrictor pharyngis. 



In stage 23 + we saw that the lateral plates of each side were still wide apart 

 except at the extreme front end of the head, where they meet ventral to the " stomo- 

 daeum." In the trunk the pronephrocceles have appeared, but ventral to this the 

 TRANS. ROY. SOC. EDIN., VOL. XLV. PART III. (NO. 23). 88 



