762 Proceedings of the Boy at Society 
fins of Selachians, Dr Dohrn abandons his former view, and gives 
a detailed account of the origin of the musculature of both median 
and paired fins. He confirms Balfour’s statement of the origin of 
the musculature of the paired fins from buds of the muscle plates, 
and extends the same to the unpaired fins, He also shows that 
muscular buds, similar to those which form the muscles of the 
pectoral fin and pelvic fin, are given off in those myotomes which 
belong to the part of the trunk between these, but afterwards 
atrophy. This fact confirms Balfour’s view, that the pectoral and 
pelvic fins were originally continuous. He then shows that muscle 
buds similar to those which form the muscles of the pelvic fins are 
given off behind the anus, forming a continuous series with those 
belonging to the pelvic fin ; and that from these the muscles of the 
anal fin are derived. From this, he argues that the anal fin has 
been formed phylogenetically from the ventral coalescence of two 
lateral folds continuous with those whose remnants form the pectoral 
and pelvic fins. The reason why coalescence has taken place 
posteriorly, and not in the region anterior to the anus, is that the 
gut has disappeared from the posterior end of the body, and not 
from the anterior pre-anal part. The gut is shown by the existence 
of the embryonic post-anal gut to have originally extended to the 
posterior extremity of the body where the original anus was, while 
after the formation of the present anus, a secondary structure, the 
posterior part of the gut disappeared, and then by the reduction in 
size of the ventral part of the tail, the two lateral folds were brought 
together and formed the median ventral fin. He further shows that 
the musculature of the dorsal median fin arises from muscle buds 
given off on each side from the dorsal ends of the myotomes, and con- 
cludes from this, since similar ventral buds form two lateral fins, that 
the dorsal fin was also orignally derived from the coalescence of two 
lateral fins. The effective cause for the coalescence here was, in his 
opinion, the folding over of the original flat plate of the central 
nervous system to form a canal. Going one step further back, he 
supposes each of these four lateral folds, composed as they are, of 
buds from successive segments, to have been originally a series of 
separate processes ; and then he points out that we have an animal 
similar to an annelid, each segment bearing a pair of notopodia, 
and a pair of ventral neuropodia. 
