58 GENERAL EMBRYOLOGY. 
Obviously, however, a system of vessels filled with fluid would be of little use 
in the general economy unless there were some means by which the fluid could be 
kept in constant movement. In the first instance this is accomplished by rhythmical 
contractions of the vessel walls, but in a short time portions of the two primitive 
stem-vessels which appear in the embryo are modified into a single propulsive 
organ, the heart, which forces the fluid, or blood, in a definite direction both 
through the body of the embryo, along the body-stalk or umbilical cord, accord- 
ing to the age of the embryo, and through the vessels in the placental villi. 
We have now to consider how the blood-vessels and blood are formed. 
Where, or how, the first blood-vessels appear in the human subject is not 
definitely known, but in 
other mammals they are 
first seen outside the body 
of the embryo in the wall 
of the yolk-sac. The outer 
layer of the wall of theyolk- 
sac consists of splanchnic 
mesoderm, and in that part 
of this layer which hes 
nearest the primitive ali- 
mentary canal a_ large 
number of the cells pro- 
liferate rapidly and, fusing 
together, form multi- 
nucleated masses of pro- 
toplasm, the ‘‘blood islands” 
of Pander. 
Soon after their appear- 
ance the blood islands 
anastomose together by 
means of nucleated pro- 
cesses which they throw 
out on all sides, and thus 
a nucleated protoplasmic 
reticulum is formed in the 
substance of the splanchnic 
mesoderm. The region in 
Fic. 44.—DEVELOPMENT OF BLOOD-VESSELS IN THE VASCULAR AREA OF which this occurs is known 
ES AD as the vascular area. The 
I. Entoderm and splanchnic mesoderm. yee : : 
Il. Proliferation of cells of mesoderm and formation of ‘‘ blood solid nucleated reticulum 
islands.” is soon converted into a 
III. Commencing differentiation of islands to form blood-vessels and system of anastomosing 
blood-corpuscles. ¥ 5 hea 
IV. Completed Becate canals, the primitive blood- 
BC Blood-corpuscles. BV, Blood-vessels. vessels, by the appearance | 
BI Blood-islands. nthi 1 i 
BV, Bod isan being trans. PN Entoderm. ee ae ‘oonment 
ormed into blood-vessels. M Mesoderm. ; 
together, whilst at the same 
time the nucleated protoplasm is transformed into cells. The cells which he nearest 
the interior separate from each other and form the primitive blood-corpuscles, whilst 
those situated externally remain connected by their margins and form the endothelial 
walls of the embryonic vessels. The fluid which fills these first-formed vessels im 
the vascular area is probably derived either from the ccelom or from the yolk-sae. 
The primitive blood-corpuscles are nucleated cells of a reddish colour; white or 
colourless blood-corpuscles appear later and those first formed are developed in the 
thymus gland. 
Nucleated red corpuscles persist and increase in number till the end of the 
second month of intrauterine life; they are then gradually replaced by non-nucleated 
red corpuscles. The majority of the nucleated red corpuscles disappear long before 
birth, but a few can usually be found in the blood of the new-born child. There is 
