MUSEUM OF COMPARATIVE ZOOLOGY. 95 
frequently occurred that, while seeking an embryo slightly more ad- 
vanced than that just studied, one was found among later killings 
which represented a stage twelve to eighteen hours younger. This 
fact has vastly increased the task of finding a more or less complete 
series of successive stages. 
III. — Summary or THE LITERATURE ON THE DEVELOPMENT OF 
ParrReD Fins IN FIsuHEs. 
In this summary an attempt is made to trace the history of the 
observations which have led to our present knowledge of the develop- 
ment of the pectoral fins in fishes. Papers containing only purely theo- 
retical and phylogenetic considerations upon the intensely interesting 
question of the origin of the vertebrate limbs are omitted, and also the 
observations which are based exclusively upon the morphological con- 
ditions of the adult forms. Hence, only such as are in the main embry- 
ological are referred to. Several papers are included which do not refer 
to fishes, but deal with the early development of the limbs in groups 
closely related to fishes. The earlier accounts of the fins by Rafi- 
nesque, Forchhammer, Rathke, von Baer, Lereboullet, and others, are 
not of much importance in considering the relation of the limb-buds to 
the germinal layers, and are consequently omitted. 
(a) Teleosts. — Oellacher (’79, pp. 141-143), in a preliminary notice, 
which has not, to my knowledge, been followed by a more comprehen- 
Sive communication upon the same subject, gives an account of the 
development of the paired fins in the Brook Trout. He maintains that 
the protovertebre in the region of the pectoral fins grow out laterally 
into a mass of cells, which in cross section of the embryo gives the ap- 
pearance of a triangular plate, lying directly upon the upper peritoneal 
layer, or somatopleure, and bounded by ectoderm above, the inner 
face of the triangle being continuous with the protovertebre. This 
lateral mass of mesoderm becomes thinner in passing forward, back- 
ward, and outward, ending in a distinct margin, while it is thickest at 
its base, where it is continuous with the protovertebr. Later, a change 
takes place in the contour of these plates, so that, instead of being 
thickest at their inner limit, where they are in connection with the 
protovertebre, they are now thickest in a region farther from the axis 
of the embryo, and in cross sections of the latter present the outline 
of a triangle, with a slightly acute angle above, a sharply acute angle 
outwards, the point of the inner angle being in continuity with the 
protovertebre. 
