144 TEXT-BOOK OF EMBRYOLOGY. 



represent the beginning of the branchial grooves and branchial arches, which are 

 exceedingly important in the development of the face and neck regions. The 

 branchial arches and grooves are the morphological equivalents of the gills 

 and gill slits in lower Vertebrates (Fishes, larvas of Amphibians). 



In an embryo somewhat further advanced (Fig. 122) the body as a whole 

 is more robust. The heart is more prominent, and this region is still larger in 

 proportion to the body than in the preceding stages. The dorsal flexure is 

 much reduced. The cephalic flexure is more marked than in the preceding 

 stages. Two other flexures have appeared the cervical flexure just caudal to 

 the head region, the sacral flexure near the caudal end of the body. All these 

 flexures together make the embryo as a whole appear crescentic in form. The 

 primitive segments are at the highest degree of their development and extend 

 from the cervical flexure to the caudal end of the body. 



The two vertical depressions in the head region, which were seen in the 

 preceding stage (Fig. 121), are more prominent here as the first and second 

 branchial grooves or clefts. Just caudal to these two other similar depressions 

 appear as the third and fourth branchial grooves. Cranial to the first groove, 

 between the first and second, between the second and third, and caudal to the 

 third are elevations which mark the first, second, third and fourth branchial 

 arches respectively. A strong process, the maxillary process, has grown 

 cranially from the dorsal part of the first arch. The main part of the arch is 

 the mandibular process. 



In a somewhat later stage (Fig. 123) further distinct changes have occurred, 

 some of which rather than leading toward the adult form of the body are de- 

 partures from it. For example, all the flexures have increased to such an extent 

 that the tail almost touches the head, the entire body being decidedly concave on 

 the ventral side. The dorsal flexure, instead of forming a concavity in the back, 

 now forms a distinct convexity and gives the back a rounded appearance. As a 

 general rule, the tail at this stage is bent to the right, but in some cases the bend 

 is toward the left. 



The branchial arches and grooves are especially prominent. The fourth 

 (and last) arch has appeared and caudal to this, the fourth (and last) groove. 

 The first three arches have enlarged and become elongated so that they almost 

 meet their fellows of the opposite side in the midventral line. The site of 

 the external ear is marked by the second branchial groove. In addition to 

 this, the anlagen of the other sense organs are apparent. The optic vesicle is 

 seen just cranial to the dorsal end of the first arch; the nasal fossa as a distinct 

 depression on the ventral side of the head cranial to the first arch. The yolk 

 sac has become so constricted at its base that it is now readily divisible into 

 the long, slender yolk stalk and the yolk sac or vesicle. 



On the side of the body, just caudal to the cervical flexure, a small protu- 



