REPRODUCTION AND DEVELOPMENT 



275 



The Visceral Pouches. In vertebrates six pairs of bays or 

 grooves form in the endodermal wall of the anterior end of the 

 archenteron (Fig. 186). In the fishes the posterior four or five of 

 these break through and on their walls develop the filamentous 

 gills. In air-breathing vertebrates, the fate of these visceral or endo- 

 dermal pouches is very different. In Man (Fig. 187), for example, 

 portions of the first or most anterior pouch become involved in the 

 development of the ear and form the tympanic cavity and the 

 Eustachian tube which connects it with the roof of the adult 



-^ 



/ /^~yTYM PANIC CAVITY 

 - EUSTACHIAN TUBE. 



^ 



i:Tm 



1 



TONSIL 



PARATHYROIDS 



THYMUS 



Fig. 187. — Fate of the Visceral pouches in Man. 



pharynx. The thyroid gland develops from the floor of the endo- 

 dermal region between the first and second pouch. The true ton- 

 sils, the thymus, and the parathyroids develop from the endoderm 

 of the second, third and fourth visceral pouches. In the floor of the 

 embryonic pharynx just behind the site of origin of the thyroid a 

 groove develops from which arise the trachea and lungs, the linings 

 of which are of endodermal origin; the circulatory and connective 

 tissues of the lungs are derived from the adjacent mesoderm. 



Nutrition of the Embryo. The development of an adult from 

 a fertilized ^gg is accompanied by a relatively enormous increase in 

 quantity of living material; furthermore, new and diflferent types of 



