DEVELOPMENT or FISH EMBRYOS 301 



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 role 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 KC1 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 



