176 W. B. CHAPxMAN 



says: " . . . there is a plexus of vessels covering the 



entire area opaca and area pellucida which connects with the 

 venous end of the heart and with the entire dorsal aorta of the 

 embryo opposite the zone of the myotomes." That the heart- 

 beat has much to do with the further development of this primi- 

 tive vascular system has been generally conceded, and Thoma 

 has formulated his three well-known histo-mechanical laws, 

 which, as translated into English by Bruce ('96, p. 266), are as 

 follows: ''The increase in size of the lumen of the vessel, or 

 what is the same thing, the increase in surface of the vessel wall, 

 depends upon the rate of the blood current." "The growth in 

 thickness pi the vessel wall is dependent upon its tension. Fur- 

 ther, the tension of the wall is dependent upon the diameter of 

 the lumen of the yessel and upon the blood-pressure." "In- 

 crease of the blood-pressure in the capillary areas leads to new 

 formation of capillaries." In order to test the validity of this 

 view, it was decided to remove the heart by surgical methods 

 before the establishment of a circulation, thereby eliminating 

 most of the mechanical factors mentioned by Thoma, and then 

 to study the further development of the vascular system in the 

 area vasculosa. 



II. REVIEW OF LITERATURE 



Roux ('78) briefly mentions seeing an area vasculosa of a five- 

 day chick in which the embryo was missing. He describes a 

 simple capillary plexus with a mesh-like arrangement, and 

 without formation of arteries and veins. In a note added later 

 ('95), he mentions seeing further cases of chick embryos in which 

 the embryo had failed to develop, but in which the sinus temi- 

 nalis had formed. He refers also, in a very indefinite way, to 

 the presence in these embryos of large vessels which corre- 

 sponded somewhat in position and in direction to the normal 

 arteries and veins. He gives no illustrations. 



Loeb ('93) tested the effect of solutions of potassium chloride 

 on Fundulus eggs, and observed that the living embryos de- 

 veloped without a heart-beat, the heart being thrown into a 

 state of tetanus by the potassium. In spite of this some vessels 



