422 AN INTRODUCTION TO ZOOLOGY 



43 hours (19-20 somites), and the third and fourth at 45 hours 

 (23 somites) and 3 days (35 somites) respectively, and in each case a 

 branchial membrane is formed. Externally the ectoderm opposite 

 each pouch becomes grooved inwards, constituting the ectodermal 

 moiety of the pouch and the membranes of all save the fourth break- 

 down for a while, thus giving rise to three transitory visceral clefts. 

 Between the pouches the mesenchyme thickens to form the visceral 

 arches, of which five can be recognised. Of these, the first or mandi- 

 bular arch lies in front of the hyomandibular pouch, and the second 

 arch, between this cleft and the next, is the hyoid arch. In these 

 arches develop a vascular system, composed of afferent and efferent 

 vessels, and the rudiments of skeletal supports, so that we have 

 produced a pharyngeal complex characteristic of a water-dwelling 

 Vertebrate. The mandibular arches extend downwards, uniting 

 in the middle line, and their skeletal elements give rise to the lower 

 jaws. 



The visceral pouches have but a short existence, and finally 

 disappear, save for certain remnants. The dorsal region of the 

 hyomandibular cleft takes part in the formation of the Eustachian 

 tube. The dorsal entoderm of the third cleft, and to a lesser extent 

 of the fourth cleft, gives rise to proliferations that constitute the 

 rudiments of the thymus gland. On the second day a small cell 

 thickening appears on the floor of the fore-gut between the bases 

 of the second gill pouches. Later it bulges downwards and then 

 becomes cut off as a closed vesicle. By the seventh day it has 

 divided into two, and finally these give rise to the thyroid gland of 

 the adult. 



In the region just behind the fourth visceral pouch the gut 

 narrows down to form the oesophagus, and on its floor in the transition 

 region a shallow groove makes its appearance on the second day. 

 This structure is well marked by the end of the third day, and its 

 hinder end has grown downwards and. broadened considerably. 

 The groove is known as the laryngeo-tracheal groove, since it gives 

 rise to both the larynx and the trachea, while the wider posterior 

 portion bifurcates and develops into the paired lungs. 



The oesophagus is only a short tube, and behind it the gut en- 

 larges slightly in the third day, indicating the future position of the 

 stomach, and then narrows down again to form the duodenal region. 

 The liver appears at the close of the second day as two outgrowths 

 just in front of the anterior intestinal portal on the ventral wall of 

 the gut, i.e. in close proximity to the ductus venosus and the ductus 

 Cuvieri. The anterior one, slightly the earlier to appear, grows 

 round these vessels upwards and to the left, while the posterior one 

 grows upwards and to the right. In the third and fourth days they 



