DEVELOPMENT OF RANA 193 



are conspicuous objects in the trunk of the frog, lying on each 

 side of the vertebrae, close to the points of exit of the spinal 

 nerves. Diverticula from the auditory vesicles grow into the 

 brain-case, and back down the canal formed by the vertebrae 

 and which contains the spinal cord. From here, the diverticula 

 of the auditory vesicle emerge through the foramina for the 

 spinal nerves and give rise to the glands of Schwammerdamm 

 (function unknown). 



The olfactory organs arise as a pair of thickenings of the 

 ectoderm, which sink in to form pits just above the mouth. 

 The cells lining these pits will give rise to the olfactory epithe- 

 lium. Behind, the pits reach the roof of the mouth and break 

 through forming the internal nostrils. 



The various regions of the brain are roughly marked out 

 even before the neural folds have closed over. The definitive 

 form of the brain is soon reached by means of foldings and 

 thickenings of its walls in certain places. 



A median ectodermal inpushing arises from the epidermis 

 of the front of the head, just above the mouth. This is the 

 hypophysis which grows back beneath the floor of the fore- 

 brain until it meets and fuses with the infundibular downgrowth 

 from the brain. Hypophysis and infundibulum together form 

 the pituitary body. 



Placodes and Lateral-line Organs. — The dorsal nerves and 

 ganglia in the region of the trunk consist of nerve- cells which 

 have been derived entirely from the neural crests. In the 

 region of the head, the dorsal nerve-ganglia are derived not 

 only from the neural crest, but also from thickenings of the 

 ectoderm at the sides of the head called placodes. Placodes 

 are proliferations of the deeper layers of the epidermis which 

 contribute cells to the underlying ganglia. The profundus, 

 trigeminal, facial, glossopharyngeal and vagus ganglia all 

 derive cells from the epidermis in this way, and the auditory 

 nerve is formed from the placode which invaginates with 

 the auditory sac. Indeed, the thickenings of the epidermis 

 which later become pushed in to form the olfactory sacs, the 

 lens, and the auditory sacs, may themselves be regarded as 

 placodes. 



