'40 Mr. R. G. Harrison on the 



nesoderm-cells tlie prominence becomes much more pro- 

 nounced, and soon the height greatly exceeds the breadth. 

 The cells which lie near the base are not so densely packed 

 as those lining the ectodermal walls. 



In the meantime the myotomes have sent out processes 

 from their ventral growing edges ; but instead of entering 

 into the fin-rudiment, as given by Kaestner (Arch. f. Anat. u. 

 Phys., Anat. Abth. Jahrg. 1892, p. 200) for Salmoand. Boyer 

 for Fitnduhis, they become greatly elongated and, growing 

 forward, give rise to the coraco-hyoid muscle, as has been 

 described by van Wijhe (Verb. d. Konink. Akad. van Weten- 

 schappen, Amsterdam, Deel 22) for Pristiurus and by van 

 Bemmelen (Anat. Anzeiger, Bd. iv.) for Lacerta. The first 

 myotome is at this stage quite rudimentary ; the second and 

 third lie entirely anterior to the fin, which is on a level with 

 the fourth, fifth, and sixtli. The anterior end of the Wolffian 

 duct is opposite the middle or posterior portion of the fifth 

 segment. The first myotome has no ventral process ; the 

 second, third, and fourth send out long strands consisting 

 entirely of cells from tlie cutis plate. These grow ventrally 

 in the soraatopleuric wall of tlie pericardial cavity. After a 

 certain time the first one atrophies, the second and third bend 

 forwards and are followed by the fourth (from the fifth 

 myotome), which grows straight forwards and slightly ven- 

 trally. The foremost one becomes attached to the base of the 

 hyoid arch by means of a tendon, the stalks connecting the 

 buds with the myotomes atrophy, and the three buds unite to 

 form a muscle which takes origin from the membranous 

 shoulder-girdle, and is attached to the urohyal. This muscle 

 is still divided into three segments, at least in young fish, in 

 which the yolk-sac is entirely absorbed. The last of these 

 buds extends for one whole segment under the pectoral fin, 

 "with the cells of which it is in close contact. Sections 

 through this, a little anterior to the point of origin of the 

 outgrowth, give such a figure as has been drawn by Kaestner 

 (fig. 32). This outgrowth is less well-defined than the 

 others, and in cross section it can often scarcely be distin- 

 guished from the mesoderm of the fin. It is not at all 

 unlikely that individual cells may detach themselves from it 

 and remain in the fin ; but it is certain that as a mass it takes 

 no i:>art in the formation of the fin-muscles. 



The sixth myotome has a ventral process, which, however, 

 does not grow forward as the others do. It ultimately 

 pinches 'itself off from the whole length of the myotome, and 

 becomes an independent longitudinal strand of muscle-fibres 

 which runs dorsal to the attachment to the fin, but which is 



