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 3-shaped headfold, with the sinus below the head 

 (c/. 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 aortse, 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 aortae gives off' a vitelline 

 artery, which is distributed over the pellucid and vascular areas of 

 the blastoderm. Round the margin of the vascular 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. 



Diiring 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 {cj. p. 204). 



Changes during the 2rd 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 broiight 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 



