126 BULLETIN OF THE 
probable that single cells, originating in the ventral and more axial 
region of the myotome, pass over into the pectoral plate. There are 
cells in this region such as Ziegler ('87) describes and designates 
* Bildungsgewebe," which exhibit active nuclear change, but I have not 
seen sufficient evidence in Fundulus to affirm their migration into the 
pectoral plates. If a migration of such elements does take place, it 
must be subsequent to the connection between the myotomes and 
pectoral plate, i. e. subsequent to the contribution by the cutis-plate, 
since prior to this there is no connection between the elements in 
question and the pectoral plate. 
The inferences which I am led to draw from my observations on 
Fundulus concerning the origin of the pectoral fin are: (1) that the 
first step is a differentiation in the somatopleure caused by cell pro- 
liferation in the region of the nephrostome; (2) that this process leads 
to the formation of the pectoral plate; (3) that a connection is formed 
between oach of the four most anterior myotomes and the pectoral plate, 
and that elements from the peripheral layer of these myotomes are con- 
tributed to the pectoral plate; (4) that, although the lateral portion of 
the head-mesoderm is continuous with the lateral plates, as its axial 
region is with the most anterior protovertebra, the head-mesoderm does 
not really appear to be concerned in the earliest formation of the pec- 
toral fin. 
VII. CONCERNING THE RELATION or THE PECTORAL PLATE AND THE 
ECTODERMAL FOLD. 
The earliest trace of a modification in the ectoderm which can be 
regarded as leading toward the formation of an ectodermal fold in the 
region of the pectoral fin, is observed in an embryo of about four days, 
and this modification (Plate VII. Fig. 46) merely consists of a more 
compact arrangement of the nuclei in the lower cell layer of the ecto- 
derm (la. sns. ec., Fig. 47) than is found in the preceding stages. ‘This 
modification, however, does not in any sense involve the superficial ecto- 
dermal layer (cta., Vig. 47); it is not of the nature of an evagination 
or folding of the layers of the ectoderm, and it does not at this stage 
modify even the external contour of the sections, But at this stage the 
mesodermal pectoral plate is already far advanced. 
An ectodermal fold cannot be seen before the fifth day ; the ectoderm 
then rises into a distinct fold (Plate VIII. Fig. 54, pli. ed.) and 
extends in a longitudinal direction, but not quite parallel, to the axis of 
