ORIGIN OF BLOOD AND ENDOTHELIUM 93 



many cases never establish a blood circulation. In other res- 

 pects these embryos may be very nearly normal and the develop- 

 ment and differentiation of their tissues and organs often pro- 

 ceeds in the usual manner though at a somewhat slower rate. 

 The heart and chief vessels are formed and the blood cells arise 

 and develop in a vigorous fashion. The heart pulsates rythmi- 

 cally but is unable to propel the body fluid since its venous end 

 does not connect with the yolk vessels and in many cases its 

 lumen is partially or completely oblitered by periblastic material 

 and nuclei which seem to be sucked into the heart- cavity from 

 the surface of the yolk. 



In these embryos without a circulation of the blood one is 

 enabled to study the complete development of the different 

 types of blood corpuscles in the particular regions in which they 

 originate. There is no contamination of the products of a given 

 region through the introduction of foreign cells normally carried 

 in the blood current. 



The actual haematopoetic value of the different organs and 

 tissues may be determined in the experimental embryos, and 

 clearly distinguished from the ordinary reproduction or multi- 

 plication of blood cells which in the normal embryo would reach 

 these organs through the circulation. 



The debated question as to the production of blood cells from 

 vascular endothelial cells may be conclusively answered, at least 

 for the species here studied. 



The results and conclusions derived from these experiments 

 may be summarized as follows: 



1. The Teleost embryo is capable of living and developing 

 in an almost normal fashion without a circulation of its blood. 

 Red blood cells may be seen to arise and differentiate in these 

 living embryos in two definite localities, one within the posterior 

 body region, and the other the blood islands on the yolk-sac. 



The blood cells remain confined to their places of origin, yet 

 they attain a typical red color and may persist in an apparently 

 functional condition on the yolk-sac for as long as sixteen or 

 twenty days. The normal embryo becomes free swimming at 

 from twelve to fifteen days, but these individuals without a 



