124 BULLETIN OF THE 
In the region of tho third protovertebra it (da. pet., Fig. 42) is only 
about two cells in thickness, and is less intimately connected with the 
protovertebra, while in the region of the fourth protovertebra the pec- 
toral plate or somatopleuric thickening (Fig. 43, so’plu.) is independent 
of the protovertebra, and is composed of but «a single layer of cells. 
The nuclei are however closely crowded in the somatopleure, and give 
evidence of active karyokinetic change. Caudad of the fifth protover- 
tebra, the somatopleure consists of a thin single layer of cells, inde- 
pendent of the protovertebre. 
It has been pointed out in the foregoing account, and demonstrated 
in the figures of Plate VI., that a connection between the pectoral plate 
and adjacent protovertebre has been established. The method by which 
this connection has been effected and its morphological significance re- 
main to be considered. It is evident that the pectoral plate is passive 
in this process, whereas elements from the protovertebr lose their for- 
mer epithelial arrangement in its peripheral layer and pass over into the 
pectoral plate. It can hardly be said that the elements in this con- 
tribution take the form of well defined cell-masses or buds, nor are they 
strictly isolated individual cells, and yet to a certain extent both condi- 
tions exist. The first step in establishing this connection consists in an 
increase in the number of cells at the outer ventral margin of the pro- 
tovertebra, at this stage more correctly designated myotome. The pe- 
ripheral or somatic layer of cells in the myotome, the cutis-plate, is 
only one cell deep (Plate VII. Fig. 47, da. mu.), excepting in the region 
where later the connection is formed with the pectoral plate. Here 
by cell proliferation the peripheral layer (cutis-plate) is augmented to 
a mass of cells (pr’f. my'tm., Fig. 47) which projects laterally in the 
direction of the pectoral plate, finally, in fact, extending over the prox- 
imal margin of the latter. This projection is especially pronounced 
in the middle of the first, second, and third myotomes. In the next 
step, the cells of those myotomic proliferations, which already le in 
contact with the pectoral plate, fuse with the latter ; that is, the ven- 
tral margin becomes continuous with the pectoral plate, as already 
described, so that the mass of cells constituting the former projection 
of the myotome is gradually reduced until there is a complete blend- 
ing of myotome (Plate VIII. Fig. 51) and pectoral plate. Subsequent 
to this connection of myotome and pectoral plate, cell proliferation is 
continued in the region of the former myotomic projection, and the 
elements thus produced seem gradually to crowd outward into the 
pectoral plate. 
