MUSEUM OF COMPARATIVE ZOOLOGY, 123 
The cell proliferation in the somatopleure seems therefore to produce a 
temporary thickening in front of the third protovertebra, but the only 
region in which this original thickening remains permanent is that of 
the nephrostome. Iam therefore led to believe that the condition in 
the second and third protovertebre is not concerned with the beginning 
of the pectoral fin, and that, if there is at this stage any trace of its for- 
mation, it must be referred to the region opposito the nephrostome. 
Shortly after the nephrostome is formed, a thickening of the somato- 
pleure by cell proliferation takes place, which is permanent, and leads to 
the development of the pectoral plate. This modification of the somato- 
pleure extends from the head-mesoderm backward through the region op- 
posite the third protovertebra. It may be said that it develops from the 
head-mesoderm in a posterior direction ; yet from the first the most con- 
spicuous portion of this thickening is not in the anterior part of the pec- 
toral region, but rather in the middle of it, first in the region of the third, 
and later opposite the second protovertebra. 
It is clear that at the time of the beginning of this thickening of the 
somatopleure, and during a brief period following, there is no connection 
between it and the adjacent protovertebrz (see Plate V. Figs. 30-34). 
In a later stage, however, such a connection is established. 
I shall now describe a stage in which the pectoral plate is well advanced, 
and is in histological connection with the adjacent protovertebree. 
(b) Contribution of Myotomic Elements to Pectoral Plates. — In front 
of the first protovertebra the indifferent head-mesoderm extends laterally, 
and diminishes to a single layer of cells (Plate VI. Fig. 39) corresponding 
to the somatopleure in the protovertebral region. This lateral somato- 
pleuric plate thickens in the region opposite the first protovertebra ; the 
thickening may be regarded as the anterior region of the pectoral plate, 
and results prineipally from a cell proliferation in the somatopleure. In 
the region of the first protovertebra there is no separation between the 
protovertebree and the lateral or pectoral plates, i. e. the protovertebra 
passes gradually over into the lateral layer, which is in no sense cut off 
from it. This lateral mesodermie plate (la. pet., Fig. 40) is therefore 
continuous anteriorly with the head-mesoderm, and axially with the first 
protovertebra ; in its relation to the coelomic cavity it represents morpho- 
logically the somatopleure. The somatopleurie thickening is still more 
pronounced in the region of the second protovertebra (Fig. 41, la. pet.), 
where it is about three or four cells in thickness. The pectoral plate is 
here so closely connected with the protovertebral mesoderm as to render 
it impossible to distinguish any sharply defined boundary between them. 
