EMBRYOLOGY 207 
of the tube in front of the first somite forms four successive swell- 
ings, the cerebral vesicles, from the foremost of which (the forebrain) 
a pair of lateral processes (the optic vesicles) grows out; near 
the end of the future head a pair of shallow pits (the auditory 
pits) is visible. The number of somites increases from four or 
five to as many as fifteen during the second day. Eventually about 
fifty are present. 
Another most important feature of the first half of the second 
day is the formation of the heart and of the principal blood-vessels. 
The whole heart is developed out of the inner or splanchnic layer 
of the mesoblast on the ventral side of the future throat. To 
understand this complicated developmental feature, we have to 
remember the G-shaped headfold, with the sinus below the head 
(cf. p. 201), and have also to resort to transverse sections. The right 
and left splanchnopleuric layers bulge inwards, and meet each other 
in the medio-ventral line, thus shutting off a space, the foregut or 
anterior end of the alimentary canal, lined with hypoblast. The 
mesoblastic portion of the walls of the right and left recesses, below 
the foregut and above the splanchnopleuric extension over the yolk- 
sac, bulge out, thicken, and become hollow; each tube being con- 
tinued forwards as an aorta, and backwards, at right angles to the 
axis of the embryo, as the vitelline vein. Thus the heart consists 
originally of a right and a left tube; the median septum, which 
separates the two, becomes absorbed, and the now single heart 
begins to beat, first with slow and rare pulsations. In front the 
two primitive aortz, into which the contractions of the heart pump 
the fluid, bend upwards round the sides of the foregut, and then run 
backwards towards the tail; each of these aortz gives off a vitelline 
artery, which is distributed over the pellucid and vascular areas of 
the blastoderm. Round the margin of the vaseular area of the 
blastoderm runs a red line, the vena terminalis, through which and 
other vessels spread over the blastodermic layer of the yolk-sac 
the blood is collected into the two vitelline veins, and by them con- 
veyed into the hinder or venous end of the heart. 
During the second half of the second day all the changes initiated 
during the first half become more advanced or completed. Besides 
the headfold, the tailfold appears; in addition, the amnion grows 
rapidly, and the allantois begins to be formed (cf. p. 204). 
Changes during the 3rd day.—This is the most eventful day of the 
embryonic chick, because the rudiments of so many important 
organs now first make their appearance. The blastoderm spreads 
over about half the yolk. The white of the egg decreases consider- 
ably, consequently the vessels of the vascular area are brought near 
the shell, and act as the chief organ of respiration. The blood 
leaving the body by the vitelline arteries is carried to the small 
vessels of the vascular area, where it is exposed to the influence 
