444 EMBRYOLOGY OF THE CIIICK. 



the heart, the legs near the tail. The eyeballs (fig. 233, OP) 

 project to a large extent, and the primitive skull begins to form 

 in the mesoblast surrounding the brain. The true vertebrfE 

 arise from the old somites by a fresh segmentation, which does 

 not follow that of the old segmentation of the muscle-plates. 

 Each vertebra is formed of parts of two somites. The vertebral 

 column now becomes supported by cartilage, and the notochord 

 vacuolated and eventually squeezed out by subsequent ossifica- 

 tion. Between tlio eightieth and hundredth hours the permanent 

 kidneys appear. The tube of the kidney appears first as a 

 diverticulum from the Wolffian duct. The kidney arises from 

 a mass of mesoblastic cells closely applied to the "Wolffian body, 

 but this mass soon breaks off from the former. From the 

 ureters there eventually grow out a number of tubules which 

 become continuous with the cells of the metanephros and become 

 the true kidney tubules. The kidneys are complete by the 

 seventh day. Lastly, during the fourth day the generati\'e 

 organs develop ; but at this time it is impossible to tell the sex, 

 for the first-formed cells, the primitive cells, are the same in both 

 male and female. The duct of the ovary, of which only one, 

 the right, as a rule, remains, is formed from the Miillerian duct, 

 that of the testes from the W^olffian ducts. The allantois now 

 receives two branches from the dorsal aorta ; these umbilical 

 arteries become very important towards the close of this day. 

 The first aortic arch, which runs in the first A'isceral fold, 

 degenerates, and a new one appears as the fourth arch. The 

 second pair also nearly disappear, and then a fifth pair come as 

 the fifth arch, ^\'e thus still get three pairs of aortic arches, 

 but placed farther back. Part of each remains to form the in- 

 ternal and external rarotids. The hepatic circulation also be- 

 comes complete, and the mesenteric veins of the alimentary 

 canal appear, and the septum of the ventricles now shuts off the 

 right and left lialf of that chamber. 



Changes during the Jifth day. — The whole yolk is now 

 enclosed in the blastoderm and about two-tliirds covered by the 



