146 



TEXT-BOOK OF EMBRYOLOGY. 



certain extent and partially hides them. The nasal fossa is deeper, and ex- 

 tending from it to the optic vesicle is a groove — the naso-optic furrow — which 

 bounds the maxillary process on the cephalic side. 



The tail (not clearly shown in the figure) is proportionately smaller. It 

 does not actually diminish in size, but the more rapid growth of the body makes 

 it appear to diminish. The limb buds are larger and a transverse constriction 

 divides the upper into a proximal and a distal portion. The corresponding 

 constriction in the lower limb bud has not yet appeared. The protrusion on the 



Branchial groove III 



Branchial arch III 



Branchial groove II 



Branchial arch II 



Branchial groove I 



Mandibular process 



Maxillary process 



Eye 



Naso-optic furrow 



Nasal pit 



Yolk sac 



Heart 



Lower 

 limb bud 



Upper Umbilical 

 limb bud cord 



Yolk stalk 



Fig. 124. — Human embryo with 28 pairs of primitive segments (7.5 mm.). Photograph. 



ventral side of the body, originally caused by the heart, is now more prominent 

 owing to the fact that the rapidly growing liver also protrudes ventrally. In this 

 particular case the yolk sac seems unusually large. The yolk stalk has become 

 enclosed for about half its length within the umbilical cord. 



After the stage just described the dorsal flexure becomes still less prominent, 

 the body of the embryo being less curved (Fig. 125). The cervical flexure 

 remains distinct, so that the head is bent at a right angle to the long axis of the 

 body. Two slight depressions have appeared on the dorsum of the embryo — ■ 

 the occipital depression just cranial to the cervical flexure, the cervical depression 



