510 EMBRYOLOGY OF THE CHICK 



vascular area is very prominent, although the sinus terminalis has al- 

 ready begun to diminish in distinctness. 



The amnion covers the entire chick, but as there is as yet little fluid 

 in the amniotic cavity, the amnion lies close to the embryo. 



The splanchnic stalk forms a narrow tube connecting yolk-sac and 

 mid-gut, but the somatic stalk has not kept even pace with the splanch- 

 nic, so that there is a ring-shaped space between the two through which 

 space the allantois projects. The allantois is connected by a narrow 

 stalk with the hind-gut just cephalad to the tail. 



m v The cranial flexure increases 



to a considerable extent as does 

 also the body flexure, so that the 

 embryo now describes a half- 

 circle. 



The muscle plates are nearly 

 rig. 302. vertical in position, extending al- 



Appendage muscles being budded off from most to the point of Separation of 

 myotomes in the European Dogfish, Pristiurus. b, j 1 i. 



muscle buds; my, myotomes. (From Kingsley SOmatOplCUre and SplanChnO- 



pleure, while just beyond this 



point of separation the somatopleure is raised to form a longitudinal 

 ridge on each side, which is called the Wolffian ridge. 



It is on this day also that the beginnings of the appendages, the 

 wings and legs, can be seen as local swellings of the Wolffian ridge. 



These arise (the wing-buds just posterior to the heart region, and 

 the hind-limb-buds just anterior to the tail) as conical or triangular 

 groups of mesoderm covered by ectoderm (Fig. 302). By the end of the 

 day the wing-buds have become elongated and narrow, while the limb- 

 buds are short and broad. 



The embryo now lies on its left side, torsion being complete to the 

 extent of ninety degrees. 



It is on this day also that a fourth gill cleft appears. The gill arches 

 become so thick now that one can scarcely see the aortic arches in any 

 of them. 



In the head region, the cephalic flexure presses the ventral surface 

 of the head so tightly against the pharynx that the head and pharyngeal 

 region must be removed and studied from their ventral aspects or little 

 can be observed. 



Figure 296 will show that the mandibular arch forms the more 

 caudal boundary of the oral depression, while on each side, the arch 

 forms an elevation, the maxillary processes, which grow mesiad and 

 form the antero-lateral boundaries of the mouth opening. 



The nasal pits form as hollow depressions in the ectoderm of the 

 anterior part of the head overhanging the mouth region with U-shaped 

 elevations surrounding them. The median limb is the naso-medial 

 process and the lateral limb is the naso-lateral process. The two naso- 



