38 Mr. E. G. Harrison on the 



DoLrn (" Studien IX.," p. 401) draws a sharp distinction 

 between the mode of origin of the musculature of the anal 

 fin and that of tlie ventral. In this he is followed by Kaestner 

 (Arch. f. Anat. u. Physiol., Anat. Abt. 1892, p. 200) . Dohrn 

 remarks that while the anal fin derives its muscles from 

 muscle-buds the musculature of the ventral originates " ohne 

 Vermittelung von Muskelknospen, direct durch Einwachsen 

 der Musculatur vom Urwirbel aus, wie sich leicht an Lachs- 

 und Forellenembryonen nachweisen liisst." I am unable to 

 confirm this statement, as my preparations, both surface views 

 and sections, show distinctly that the muscle-buds which 

 grow into the ventral fins are similar to those which enter the 

 anal, except that the latter are considerably larger. 



At this stage nerve-fibres from the spinal nerves of corre- 

 sponding segments which give off muscle-buds may be 

 detected in the fin. This is a very much earlier stage than 

 the earliest at which nerves could be seen in the median fins. 



The mesenchyme proliferates rapidly, while the ectoderm 

 is raised into a fuld which projects from the ventro-lateral 

 surface of the body, parallel to its long axis. The mesen- 

 chyme forms a compact mass lying under the epidermis ; the 

 region next to the somatopleure is filled with less densely 

 packed tissue. The muscle-buds project far into the fin, but, 

 unlike those in the paired fins, are separated from the epi- 

 dermis by a layer of mesenchyme. Very soon the buds 

 disintegrate. 



The region in which the first steps towards differentiation 

 of muscle first appear is the space previously occupied by tiie 

 muscle-buds. From the very first there are no traces of 

 metamerism in this muscle, althougli it is safe to assume that 

 the cells from the buds take part in its formation. About 

 the same time at a corresponding position on the opposite or 

 inner side of the fin a similar differentiation takes place. It 

 is not so likely that cells from the muscle-buds take part in 

 this. Between these two muscle-layers the cartilaginous 

 skeleton has by this time appeared. The development of the 

 skeleton has received such thorough treatment at the hands 

 of Wiedersheim and others as to render further mention of it 

 here unnecessary. 



The fin rotates so that its line of attachment to the body 

 makes an angle of about forty-five degrees with the long axis 

 of the body. The inner muscle, of which the beginning 

 was described above, becomes the protractor or abductor 

 profundus, and the outer the retractor or adductor profundus. 

 The superficial muscles develop before the twisting of the 

 fin takes place. They are formed through differentiation of 



