220 ESSENTIALS OF ZOOLOGY 



formed become deeply folded, and sensory cells are distributed in 

 the epithelial lining. 



The ear originates in the individual as a thickened plate of periph- 

 eral ectoderm on each side of the hind brain region of the head. 

 These plates invaginate (fold in), forming an auditory vesicle or sac 

 on each side and are later freed from the external ectoderm. Each 

 becomes a membranous lahy7^inth of the inner ear, which includes the 

 sensory cochlea and the semicircular canals. The middle ear develops 

 later from the first gill pouch forming a tympanic cavity. The 

 ossicles or ear bones follow later from the mesoderm. The connection 

 of this pouch to the pharynx is retained and becomes the Eustachian 

 tube on each side. The tympanic membrane develops as the meso- 

 dermic sheet between the outer extremity of the gill pouch and the 

 depression of the external cleft, which becomes external meatus. The 

 external ear arises as a mesodermal outgrowth beneath the skin. 



The taste-buds develop in the epidermis of the tongue and else- 

 where in some forms. Thus they are ectodermal. 



Circulatory System. — This system develops in the mesoderm. The 

 formation of the earliest blood vessels has not been studied in man 

 because they have already formed in the earliest embryos critically 

 studied. But in cats and other vertebrates they are formed from 

 small spaces which develop among the mesenchymal cells. These 

 spaces extend and fuse, usually developing along the longitudinal 

 axis. The heart develops as a pair of amnio-cardiac vesicles in the 

 mesoderm, one on each side of the wall of the foregut, which at that 

 time has not closed vent rally. As the digestive tube closes ventrally, 

 the vesicle from each side moves ventrally until it meets its fellow. 

 By this time each vesicle has extended lengthwise to become a tube 

 which is the omphalomesenteric vein. The anterior portions of these 

 two cavities join to form the tube which is the primitive heart. This 

 tube then bends on itself with one limb of the fold becoming the 

 atrium, which receives blood from the veins, and the other limb of 

 the fold becoming the ventricle and bulbus, which deliver blood to 

 the arteries. This represents the primitive condition as found in the 

 two-chambered heart. As development progresses a septum forms in 

 the atrium dividing it into right and left chambers and similarly an 

 interventricxdar septum forms to produce right and left ventricles. 

 These septa are completed at about the time of birth. 



In the embryo, the gill arches are supplied with branches of the 

 ventral aorta, which leads anteriorly from the heart. These branches 



