PAIRED FINS WITH THEIR NERVES, IN LEPIDOSIREN AND PROTOPTERUS. 625 



I could find no certain evidence for the existence of the occipital myotome iv, which 

 Semon found to be the anterior occipital myotome in Ceratodus. From Sewertzoff's 

 account it appears that in his specimens Mv was also present. It is, however, difficult 

 to identify the individual myotomes at early stages, owing to the lack of a fixed point 

 of comparison. The nearest approach to such a fixed point is found in the anterior 

 pronephrostome. The nephrotome separates from the myotome between stages 

 20 and 24, but even after this the nephrostomes do not seem to change their 

 positions relative to the myotomes. From stage 29 onwards it is easy to recognise 

 the different myotomes by their relation to the developing coraco-hyoid, etc. (as 

 described later), and by their innervation. In six embryos of Lepidosiren distributed 

 over stages 29, 30, 31, and 31 +, the anterior pronephrostome was found in every case 

 to be beneath Ml (fourth metotic). It is therefore constant in position from stage 29 

 onwards. If we assume that it is under Ml in all younger embryos too, it follows that 

 in one specimen of stage 26, in which this was the fifth metotic, M.iv was present, and 

 in one of stage 25 + , in which it was the third metotic, My was the anterior occipital 

 myotome. I give the observations for what they are worth.* 



Comparing the foregoing account of the occipital myotomes with Miss Platt's 

 account of Necturus, we see that they agree in that the single occipital arch is developed 

 in the myoseptum between the third and fourth metotic myotomes. It has already 

 been shown that as regards the development of the occipital region of the skull these 

 Dipnoi resemble the Amphibia very closely, especially as regards its protometameric 

 nature (the skull may become auximetameric in adult Protopterus by synchondrosis of 

 the anterior neural arches with the occipital arch — Furbringer). The relations of the 

 occipital and post-occipital nerves and myotomes to the occipital arch give no reason 

 to suppose that the simple condition of this part of the skull in Lepidosiren and 

 Protopterus is not truly primitive — that is, we suppose that the phylogeny of the 

 Dipnoi, as of the Amphibia, did not include a stage with a complex occipital arch as at 

 present found in Elasmobranchs, etc. 



He. Derivatives of the Anterior Trunk Myotomes. 



At about stage 29 the myotomes begin to put out their ventral processes (with the 

 exception of M.x, which does not form one). The series of ventral processes is inter- 

 rupted by the presence of the pronephros, the nephrostomes of which, with the tubules 



* In Ceratodus embryos Semon found that the occipital myotomes reach up to directly behind the auditory 

 vesicle. In one specimen, however, in which M« was absent, there was a corresponding gap between the auditory 

 vesicle and the anterior occipital myotome x. In Lepidosiren and Protopterus there is a gap, about equal to the length 

 of a myotome, between M.x and the auditory vesicle, which might be thought to represent the position of Mw, In the 

 individual of stage 26, in which the anterior pronephrostome is under the fifth metotic myotome, this gap is as large 

 as usual. 



