Albert C. Eycleshymer 311 
not restricted to any particular locality. In the vicinity of the myotomes 
they are found for the most part in the intermuscular mesenchyme. An- 
terior to the myotomes they are irregularly scattered and although a few 
are found in the outer portion of the dermis, they are for the most part 
located in the deeper mesenchyme. 
A study of the formation of pigment in the mesenchymal cells shows 
that it first appears in the immediate vicinity of the nucleus, and from 
this locality extends into the cytoplasmic processes. The pigment gran- 
ules are at first separate and distinct, but as the cells become more deeply 
pigmented, the granules become less distinct. It is thus possible in a 
general way to differentiate between the younger and the older chroma- 
tophores. Fig. 1 is taken from a section through the head of a larve of 
the above length, and shows two stages in the growth of the chromato- 
phore. The deeper cell (a) represents one of the first stages in the 
formation of the pigment. The pigment is here largely confined to the 
region of the nucleus, having extended but slightly into protoplasmic 
processes. The cell nearer the surface (b) represents a later stage in 
which the pigment has extended farther into the protoplasmic processes. 
Other sections show various stages in the formation of pigment from its 
first appearance in the region of the nucleus to its extension into all the 
protoplasmic processes of the cell. 
The epidermis at this time is made up of different kinds of cells. 
The first and most numerous are the ordinary polyhedral cells, which 
contain fine yolk granules. The second and less numerous are the large 
oval or spherical cells which contain very large yolk granules, and prob- 
ably form the unicellular glands. The third and least numerous are 
certain cells which possess more or less extended cytoplasmic processes 
and which, from their granular contents, staining capacity, and general 
form, closely resemble mesenchymal cells. Such a cell is shown in Fig. 
2, c. lying among the epidermal cells. Whether these cells are modified 
epithelial cells or are mesenchymal cells, which have wandered into the 
epidermis at some earlier stage, cannot be definitely determined. As will 
be seen later cells of this type give rise to one group of epidermal chro- 
matophores. 
Larve 15-16 mm.—In the preceding stage but few of the chromat- 
ophores were at the surface of the dermis, but in the present stage large 
numbers of them have reached its outermost surface and through their 
widely branching processes form an open meshwork. These superficial 
chromatophores are most numerous over the dorso-lateral surfaces of the 
head, but they are also scattered along the body, being confined for the 
most part to an irregular dorso-lateral band which extends from the re- 
gion of the gills to the posterior limb buds. 
