CHANGES DUEIKG THIED DAY, 417 



the S being venous and the left-hand curve arterial. The venous 

 bend has two bulges, the future auricles, the ventricles appear- 

 ing at the other bend. The first true circulation of the blood 

 takes place during the close of the second day. The heart at 

 this stage gives rise to tioo aortm ; these later unite into one 

 trunk behind the head, and towards the tail this trunk gives 

 rise to two vitelline arteries, which run to and are lost in the 

 vascular area. Here two veins arise from the capillary net- 

 work, the vitelline veins, which run back to the heart. In this 

 stage we may find two, or even three, pairs of aortic arches. 

 The whole embryo is raised up from the blastoderm, and the 

 head end becomes bent. In the region of the middle somites 

 there appears a small mass of cells seen in a transverse section ; 

 this is a ridge, the primitive excretory or Wolffian duct, which 

 becomes hollow, and connected with the embryonic excretory 

 organs, the Wolffian bodies. 



Changes during the third day. — One of the most noticeable 

 things in an egg on the third day of incubation is the dis- 

 appearance of much of the white, which has been absorbed 

 directly by the blood-vessels and indirectly by the diminishing 

 yolk. The vascular area has much increased, and not only now 

 supplies the embryo with food from the yolk, but by virtue 

 of its external position (lying against the shell) it takes part in 

 the respiration of the embryo. The amnion forms a complete 

 investing membrane, and the whole embryo comes to lie on its 

 left side. The head region also becomes much bent under, 

 forming the cranial flexure, the second brain vesicle now being 

 in front, the first bent below. On examining a section through 

 the brain region we shall see that there appears a process on each 

 side of the summit of the brain : these lateral processes are the 

 origin of the cranial nerves (fig. 200, O). They are formed 

 as paired outgrowths, which afterwards shift their attachment 

 to the floor of the brain. Similarly the spinal nerves now arise 

 and become divided ipto three parts — a root, a ganglion, and 

 the distal nerve. At this period the eyes are formed by the 



2d 



