78 
ZOOLOGY: G. A. BAITSELL 
Proc. N. a. S. 
very lightly in the early stages by any of the numerous methods tried, is 
difficult to detect. This ground substance can be demonstrated around 
the notochord soon after it is formed and shortly before the embryo has 
reached the tail bud stage. A little later this material, which is to form 
a part at least of the connective tissues, surrounds the medullary cord and 
a layer of it, following the body wall, extends ventrally on either side and 
in time completely encircles the body cavity. The formation of this matrix 
around the notochord occurs before there is any syncytium of the mesen- 
chyme cells in this region. In fact, at this stage, the cells here present 
have not yet assumed the shape of typical mesenchyme cells which later 
appear. It is evident, therefore, that this primitive ground substance of 
connective tissue has arisen as an intercellular secretion of the embryonic 
cells ^ and not by a syncytial fusion of cytoplasm. 
2. The ground substance having been formed, cells begin to move into 
it and wander through it. These are spherical at first but, as they move 
through the ground substance, they soon change into various shapes, 
becoming stellate, spindle-shaped, etc. The study of the sections shows 
that, in general, individual cells do not separate from the cell masses and 
move out into the various cavities and open spaces of the embryo until 
after the formation of the secreted ground substance which is the fore- 
runner of the connective tissues. The present work, therefore, adds 
additional evidence in support of the conclusions which Harrison^ and 
others have reached with regard to the stereotropism of cells in tissue cul- 
tures. Cells, whether in tissue cultures or in the developing embryo, 
have need of a supporting framework of some kind in order for migration 
from the main masses to take place. 
3. The connective tissue fibers begin to arise in the ground substance 
soon after it has formed. In some cases, they form in the ground sub- 
stance in regions which are free from cells so that it is very clear that they 
have not arisen by any intracellular action. Instances can be noted in 
which it appears that the fibers are formed by a gradual transformation 
of the ground substance, first, into a delicate net-like structure and then 
into the long fibers which are typical of connective tissue. In its morpho- 
logical features, at least, this process gives evidence of being identical with 
the one previously observed in the transformation of the plasma clot. 
4. In other regions of the ground substance into which the cells have 
wandered, the formation of the fibers can also be shown to be due to 
changes in the ground substance and not to a sloughing off of the cell 
protoplasm. A differential stain, such as Mallory's connective tissue 
stain, shows very clearly the limits of the cell cytoplasm and the beginning 
of the ground substance. In many cases the paths of the cells can be 
traced through the ground substance and the possibility arises that, in 
their movements through the ground substance, they may give off secre- 
tions which play a part in the final chemical constitution of the fibers. 
