DEVELOPMENT or FisuH EmsBryos B01 
spread over the surface of the vessels, in consequence per- 
haps of a chemical stimulus. Whatever may be the cause 
which compels the chromatophores to creep upon the blood- 
vessels, my observations certainly show that the distribution 
of the chromatophores, and therefore the marking of the 
yolk-sac, is dependent upon the arrangement of the blood- 
vessels. I will not enter upon this point in greater detail 
here, as a separate paper on this subject will appear in the 
Journal of Morphology. I wish only to point out that this 
is the first case, to my knowledge, in which the physiological 
explanation of the marking of an animal organ has been 
found. 
7. Nowhere will the mystic find a richer field of unex- 
plainable purposefulness than in the developmental history 
of the higher animals. In these everything apparently comes 
into being at the right time and at the right spot, as though 
each element knew what rdle it had to play in the whole. 
The heart also begins to beat, apparently, just at the right 
moment; and I always had the idea—and others will per- 
haps have shared it—that if the activity of the heart were 
interfered with, development would soon cease. Our experi- 
ments, however, show that, if we do not consider the extreme 
cases, the development of an embryo in a KCl solution can 
keep on normally for three days after the formation of the 
heart, even though no circulation is established. This lati- 
tude for the time of the beginning of the heart-beat is, when 
compared with the total time of development, very far from 
the precision expected of a clock-work. 
8. In conclusion I wish to emphasize what seems to have 
been definitely established by these experiments, and what is 
yet to be determined by further experiments. I consider it 
certain that the origin, the pathway, and the branching 
of at least the larger blood-vessels are independent of the 
blood-pressure. For this reason it is possible for a vascular 
Digitized by Microsoft® 
