114 Cc. V. MORRILL 
distinguishable earlier while in most the two appeared at about 
the same time. Glaeser seems to think the axial cartilage ap- 
pears before the peripheral, while Wendelstadt describes activity 
in the periosteum leading to the formation of peripheral cartilage 
before there is any outgrowth of cells from the marrow cavity 
to form axial cartilage. In the experiments on Diemyctylus 
the presence of an epiphysis may have had a modifying effect 
on the order of appearance. This point is probably not of any 
great importance. 
Soon after the appearance of axial and peripheral cartilage 
the tissue of the bud which has formed distal to the femur, begins 
to show signs of differentiation. This tissue as mentioned 
previously is at first composed of apparently indifferent cells. 
Cartilaginous masses now appear in it and, since they are formed 
in the same manner as in normal development, they may be 
spoken of collectively as embryonal cartilage following Glaeser 
(10). In figure 2, a mass of this cartilage (Hm.C.) can be seen 
lying distal to the old epiphysis (Hp.f.). It is at first entirely 
distinct from the axial cartilage. As the limb bud enlarges 
rapidly the growth of the embryonal cartilage keeps pace with it, 
the cartilage extending right into the growing tip (fig. 3, Hm.C.) 
In this way the skeleton of the new leg and foot is blocked out 
in cartilage at a young stage, as the early experimenters found. 
The formation of new skeletal parts is due chiefly to concentra- 
tions of cells in situ rather than to growth from the first formed 
mass or masses. ‘There is, it is true, a continuous substratum or 
core of tissue running through the center of the bud from which 
the skeleton arises but it does not give rise to one or two con- 
tinuous bars of cartilage. There are always interruptions at 
points where joints are to be formed. 
During the growth of the embryonal cartilage, the axial 
cartilage continues to enlarge. This is due in part to growth 
at the expense of the old epiphyseal cartilage. The latter 
remains, however, for some time partly imbedded in the axial 
cartilage (fig. 3, Hp.f) where it can be distinguished by its different 
6 The space seen in figure 2, between the embryonal cartilage and the old 
epiphysis is probably an artifact. 
