416 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



vesicles known respectively as the fore, mid, and hind-brain. 

 From the fore-brain arise the cerebral hemispheres (cf. Fig. 38, 

 p. 65), with the olfactory lobes at their anterior ends, and the 

 diencephalon; from the mid-brain, the optic lobes; and from the 

 hind-brain, the cerebellum and the medulla. The ventricles of the 

 brain and their connections represent a survival of the cavity of 

 the neural tube, which thus extends as a closed canal from end 

 to end of the adult nervous system. 



The spinal nerves arise as outgrowths from the region of the 

 central system, the nerve fibers in the ventral roots growing from 

 cells in the neural tube, the dorsal roots from the dorsal-root gangha 

 that are formed from the neural crests (Fig. 216 B). The devel- 

 opment of the cranial nerves is more complex, but in all cases the 

 nerve fibers that make up the nerves grow outward from nervous 

 cells at some point in the nervous system. The notochord is 

 replaced by the vertebral column, which forms around the noto- 

 chord as the latter ceases to develop and becomes resorbed. Ref- 

 erence has been made to the fate of the neurenteric canal and the 

 formation of the digestive tract by union of the stomodseal and proe- 

 todaeal invaginations with the enteron. The lungs and larynx 

 originate as a median outgrowth on the ventral wall of the enteric 

 cavity, which is at first single and becomes double as the lungs 

 develop. The liver and pancreas (cf. Figs. 215 E and 217) likewise 

 originate as evaginations of the enteron, at first separate and later 

 united by their common duct. The urinary bladder, which also 

 arises as an evagination from the ventral floor of the enteron, is 

 a rudiment in the tadpole, developing further at the time of meta- 

 morphosis. Thus, the development of lungs, hver, pancreas, and 

 urinary bladder is such that each of these organs is Hned by cells 

 of endodermal origin, which are covered externally by tissues of 

 mesodermal origin, such as connective tissue, muscle fibers, and 

 peritoneum. With the dmiinution of the yolk-laden cells {cf. Fig. 

 216 C), the cleft in the mesoderm that forms the ccelome becomes 

 extended ventrally as a spacious coelomic cavity. The heart 

 appears as a tubular structure (Fig. 217), which begins to pulsate 

 before the circulation is established ; blood vessels, which are differ- 

 entiated in their definitive positions, gradually become associated 

 with the heart as a circulatory system, which resembles that of a 

 fish in the tadpole stages, and assumes the adult form only at 

 the time of metamorphosis. The kidneys and reproductive organs 



