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TEXT-BOOK OF EMBRYOLOGY. 



grooves are still prominent. The first groove, of which the dorsal part 

 marks the site of the external auditory meatus, is at this time particularly 

 well developed. The eye is a stronger feature than in the preceding stage. 

 The distinct depression in front of the first arch is the nasal fossa. The 

 limb buds are larger than in the 4 -mm. embryo. The general curvature of 

 the embryo is so sharp at this stage that the rudimentary tail is almost in 

 contact with the head. 



In Fig. 88, showing an embryo of 7.5 mm. with 27 primitive segments, 

 the head is somewhat larger in proportion to the body. This character 



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 



Liver 



limb bud 

 FIG. 88. Human embryo with 28 primitive segments (7.5 mm.). Photograph. 



Umbilical 

 cord 



Yolk stalk 



becomes accentuated as development proceeds and is especially noticeable 

 up to the time of birth. The cervical and sacral flexures are still sharp, but 

 the dorsal flexure is not quite so prominent. From now on, the body becomes 

 more nearly straight. The rotundity of the ventral side of body is due to 

 the heart and liver, the two organs now lying close together. The branchial 

 arches are not actually smaller but appear less prominent. The second 

 arch has enlarged and grown back over the third and fourth, partially hiding 

 them. The limb buds' are larger; and the fore-limb bud now shows a trans- 

 verse constriction dividing it into a proximal and a distal portion, the latter 

 being the rudiment of the hand. 



