418 The Submaxillary Gland 
which give the stimulus for the growth of new capillaries. It must be 
obvious that the principal factors that govern organic growth are resi- 
dent in the cells rather than in the blood-vessels, as is indicated by their 
behavior in the embryo before the vascular system is laid down. We 
have still much to learn concerning the factors that arrest the growth of 
organs when they reach the adult type, but there is little doubt that these 
phenomena are expressions of cellular rather than vascular activity, 
since the vascular system maintains far beyond the usual period of 
growth, its power of progressive development. It is, so to speak, always 
in a state of unstable equilibrium, in which both progressive and regres- 
sive changes are possible. Certain facts in the development of the blood- 
vessels of organs have already been demonstrated, the most important of 
which is that the intrinsic blood supply of organs usually marks out the 
paths along which the units of structure that compose it have developed. 
And by following the gradual increase in complexity through a series 
of injected embryos, the succeeding changes from the simple embryonic 
to the adult form can be easily demonstrated. It is important to trace 
these changes not only for the light they shed on the development of the 
vascular system, but because many obscure features in the structure of 
organs are elucidated when the mechanics of their development are 
known. 
Excepting certain radical modifications that take place at the time 
when the embryo ceases to receive its nourishment from the maternal 
blood sinuses and independently undertakes the aération of its own blood, 
it has been shown that the conditions of the systemic circulation in 
embryos at term approximate very closely that in adult life. At the 
same time considerably less differentiation occurs in the intrinsic circu- 
lation of the individual organs than in the larger vessels of the general 
circulation. At the time of birth the structure of most organs is well 
developed, and the changes which take place are usually quantitative 
rather than qualitative. Accordingly, the material for this research 
was obtained from the submaxillary gland of injected embryo pigs, and 
the results were subsequently shown to conform to the conditions found 
in the glands of dogs and human beings. 
METHODS, 
The technique for the injection of embryos used in the study of the 
angiogenesis of the submaxillary has been described in detail in another 
2Flint, Welch, Festschrift, 1900, and Reports of the Johns Hopkins Hospital, Vol. 
IX., 1900. 
