330 OUTLINES OF CHORDATE DEVELOPMENT 



arches; each is covered externally by ectoderm, and internally 

 and laterally by endoderm. Five visceral arches are thus 

 marked out. The first is in front of the hyomandibular pouch, 

 between this and the mouth, and is known as the mandibular 

 arch. The second, or hyoid arch, lies between the hyoman- 

 dibular and the second visceral pouches, while the remaining 

 three branchial arches (third to fifth visceral arches) lie pos- 

 terior to the second, third, and fourth visceral pouches. Like 

 the pouches these diminish in size and importance posteriorly, 

 the last (fifth) being only a slight and transitory vestige. 



The vertical fusions of ectoderm and endoderm along the 

 outer borders of the visceral pouches are interrupted just below 

 the upper ends of the pouches, so that dorsal and ventral 

 portions of each fusion are to be distinguished (Fig. 130, B). 

 The surface of the head becomes depressed in the lines of fusion, 

 so that a series of vertical grooves marks externally the dis- 

 position of the visceral arches and pouches. 



The second visceral pouch (first branchial) is the best de- 

 veloped, and about the end of the second day both upper and 

 lower fusions are perforated as the vestiges of a gill cleft; these 

 perforations close during the fourth day without leaving any 

 trace. In the third pouch a gill cleft is similarly formed and 

 closed a little later. In the first pouch (hyomandibular) only 

 the dorsal fusion is perforated (spiracular cleft) shortly before 

 the perforation of the second pouch. No cleft appears in the 

 fourth pouch, only the dorsal portion of which fuses with the 

 ectoderm. 



After the fourth day the visceral pouches become reduced. 

 The first undergoes a change in function and takes an essential 

 part in the formation of the tympanic cavity, as described 

 above. For the most part the other visceral pouches finally 

 disappear, but from parts of their epithelial walls the thymus 

 and the post-branchial bodies are derived. The thymus is 

 chiefly derived from part of the dorsal epithelium of the third 

 visceral pouch, but the fourth contributes to a small extent. 

 A transitory anterior portion of the thymus is derived 

 from the epithelium of the second pouch. The thymus be- 





