Development of the Fins of Teleosts. 39 



the mesenchyme cells which lie between the deeper muscles 

 and the epidermis. It is extremely improbable that cells 

 from the muscle-buds take any part in their make up. The 

 muscles and skeleton grow forward in the body-wall between 

 the ventral ends of the myotomes, so that eventually only a 

 very small portion of each of the muscles lies in the free 

 extremity. In embryonic stages these muscle-masses are 

 continued distally, without sharp dividing line, into the 

 mother-cells of the dermal rays. 



The Pectoral Fin. — The pectoral fin diverges from the 

 primitive type more than the other fins, both in its definitive 

 structure and in its course of development. It develops 

 considerably earlier than the others, and lack of histological 

 differentiation of the tissues at that time renders its study 

 more difficult. 



The first trace of this fin is to be seen in a thickening of 

 the* somatopleure ; the thickening of the ectoderm and its 

 fold arise later. This is in accordance with Boyer's obser- 

 vations on Fundulus (Bull. Mus. Comp. Zool. vol. xxiii. 

 no. 2). The thickened portion of the somatopleure is not 

 confined to the " pectoral plate," but extends to the portions 

 of the splanchnopleure, on the same level, and through the 

 nephrostome to the Wolffian duct. This thickened portion of 

 the peritoneum is due to the cuboidal or columnar character 

 of the epithelium composing it. Anteriorly, laterally, and 

 posteriorly to it the cells flatten out. There is, just anterior 

 to it, a portion of the body-wall in which are numerous 

 mesenchyme cells derived from the head mesoderm. Ziegler 

 (Arch. f. mikr. Anat. Bd. xxx.) has regarded this as the 

 rudiment of the fin. Study of the later stages shows, how- 

 ever, that this region lies completely anterior to that in which 

 the fin develops. 



At a somewhat later stage the cells belonging to the pectoral 

 plate become much more distinctly columnar than the others, 

 and, multiplying rapidly, soon become several layers thick, 

 and are much more densely packed than those lying anterior 

 to them. A thickening of the epidermis now takes place, 

 which, unlike that of the ventral fin, consists in an increase 

 in size of the individual cells, and not in a multiplication of 

 the same. At the crest of the prominence which the prolife- 

 ration of the mesoderm has caused, the ectoderm is thrown 

 into a fold parallel to the axis of the body and extending 

 through three somites. In cross section the structure is 

 triangular ; the somatopleure, which extends out over the 

 yolk-sac, is its base, and is nearly horizontal. In profile the 

 crest is semicircular. Through rapid proliferation of the 



