DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



113 



An embryo of 4 mm. is shown in Fig. 86. All the flexures are accen- 

 tuated, so that the head and tail are close together. The fourth branchial 

 arch has appeared behind the third groove and a fourth groove behind the 

 fourth arch. The small structure behind the fourth groove may be the 

 rudimentary fifth arch. The arches diminish rather uniformly from the 

 first to the last. The rudiment of the eye is visible on the side of the fore- 

 brain region as a circular eminence surrounded by a slight groove. The heart 



Cervical Cervical 



depression flexure 



Dorsal flexure 



Branchial arch IV 

 Branchial groove III 

 'Branchial arch III 

 Branchial groove II 



Branchial arch II 

 Branchial groove I 



Branchial arch I 

 Mandibular process 



- Maxillary process 



-Eye 



Nasal pit 



Heart 

 Yolk stalk 



Lower limb bud 



Primitive segments 



Upper limb bud Liver Sacral flexure 



FIG. 87. Human embryo with twenty-seven primitive segments (7 mm., 26 days). 



Mall. 



protuberance is strikingly prominent. Certain new features have appeared 

 at 'this stage, the limb buds. The fore-limb bud is a rounded eminence 

 opposite the anterior part of the dorsal flexure; the hind-limb a similar 

 structure opposite the sacral flexure. The limb buds, as tar as surface 

 appearance goes, are Simply outgrowths from the body wall starting as small 

 rounded eminences which, as development proceeds, become larger and finally 

 differentiated into the various parts of the extremities. 



In a 7-mm. embryo described by Mall (Fig. 87), the flexures are slightly 

 more accentuated than in the 4-mm. stage. The branchial arches and 



