ZOOLOGY: C. R. STOCKARD 
557 
to study the complete development of the different types of blood cor- 
puscles 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 stream. 
The actual power to form blood cells possessed by the different organs 
and tissues may be determined in the experimental embryos having no 
blood circulation. And the true haematopoetic function is thus clearly 
contrasted with the ordinary reproduction or multiplication of blood 
cells which might take place within the tissue spaces of such organs in 
the normal embryo. 
The debated question regarding the production of blood cells from 
those cells lining the blood vessel wall may be convincingly answered, 
at least for the species here studied. 
The results and conclusions derived from these experiments may be 
summarized as follows: 
1. The fish embryo is capable of living and developing in an almost 
normal fashion without a circulation of its blood; this fact was first 
recorded by J. Loeb in 1893, yet this is the initial study of blood and 
vessel formation in such embryos. Red blood cells may be seen to arise 
and differentiate in these living embryos in two definite localities; the 
one within the posterior body region, and the other the blood islands 
on the yolk. 
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 to twenty days. The 
normal embryo becomes free swimming at from twelve to fifteen days, but 
individuals without a circulation never hatch although they may often 
live for more than thirty days. 
All recent investigators have claimed that there are no blood islands 
present on the bony fish yolk-sac. Yet the presence of such islands is 
readily demonstrated in living Fundulus embryos, in the normal as well 
as in those with no circulation. 
2. The plasma or fluid in the embryos which fail to develop a circu- 
lation begins to collect at an early time in the body cavities. The peri- 
cardium becomes hugely distended with fluid, as well as the lateral 
coelomic spaces and the Kupffer's vesicle at the posterior end of the 
embryo. The great distension of the pericardium due to this fluid 
accumulation pushes the head end of the embryo unusually far away 
from the surface of the yolk. The heart is thus often stretched into a 
long straight tube or string leading from the ventral surface of the 
head through the great pericardial cavity to the anterior yolk surface. 
