252 ORGANS DERIVED FROM THE GERMINAL LAYERS. 



structure of the medullary plate in Amphibia may be more primitive 

 than that in other types \ 



Formation of the organs of special sense. The more important 

 parts of the organs of smell, sight, and hearing are derived from the 



epiblast; and it has been as- 

 jviTrf serted that the olfactory pit^ 



ep optic vesicles and auditory 

 pit take their origin from a 

 special sense plate, continuous 

 at first with this medullary 

 plate. In my opinion this 

 view cannot be maintained. 



In the case of the group 

 of forms in which the epiblast 

 is early divided into nervous 

 and epidermic layers, the 

 former layer alone becomes 

 involuted in the formation 

 of the auditory pit and the 

 lens, the external openings 

 of which are never developed, 

 while it is also mainly con- 

 cerned in the formation of the olfactory pit. 



Fig. 192. Tbansveese section through the 



CEPHALIC REGION OF A YOUNG NeWT EMBRYO. 



(After Scott and Osborn.) 



In.liy. invaginated hypoblast, the dorsal part 

 of which wiU form the notochord; ep. epiblast 

 of neural plate; sp. splanchnopleure ; al. ali- 

 mentary tract ; yk. and Yhy. yolk-cells. 



Summary of the more important Organs derived from the three ger- 

 minal layers. 



The epiblast primarily gives origin to two very important parts of 

 the body, viz. the central nervous system and the epidermis. 



It is from the involuted epiblast of the neural tube that the 

 whole of the grey and white matter of the brain and spinal cord 

 appear to be developed, the simple columnar cells of the epiblast 

 being directly transformed into the characteristic multipolar nerve 

 cells. The whole of the sympathetic nervous system and the peri- 

 pheral nervous elements of the body, including both the spinal and 

 the cranial nerves and ganglia, are epiblastic in origin. 



The epithelium (ciliated in the young animal) lining the canalis 

 centralis of the spinal cord, together with that lining the ventricles 

 of the brain, is the undifferentiated remnant of the primitive 

 epiblast. 



The epiblast also forms the epidermis ; not however the dermis, 

 which is of mesoblastic origin. The line of junction between the 



1 A parallel to the unpaired medullary plate of most Chordata is supplied by the 

 embryologically unpaired ventral cord of most Gephyrea and some Crustacea. In these 

 forms there can be little doubt that the ventral cord has arisen from the fusion of 

 two originally independent strands, so that it is not an extremely improbable hypothesis 

 to suppose that the same may have been the case in the Chordata. 



