304 ORGANS DERIVED FROM THE GERMINAL LAYERS. 



quite possible that the structure of the medullary plate in 

 Amphibia may be more primitive than that in other types 1 . 



Formation of the organs of special sense. The more 

 important parts of the organs of smell, sight, and hearing are 

 derived from the epi- 

 blast ; and it has been 

 asserted that the olfact- 

 ory pit, 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 main- 

 tained. 



In the case of the 

 group of forms in which 

 the epiblast is early divi- 

 ded into nervous and epi- 

 dermic 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 concerned 

 in the formation of the olfactory pit. 



0.1 



FIG. 192. TRANSVERSE SECTION THROUGH 

 THE CEPHALIC REGION OF A YOUNG NEWT EM- 

 BRYO. (After Scott and Osborn.) 



In.hy. invaginated hypoblast, the dorsal part 

 of which will form the notochord ; ep. epiblast 

 of neural plate ; sp. splanchnopleure ; a/, ali- 

 mentary tract ; yk. and Y. hy. yolk-cells. 



Summary of the more important Organs derived from the three 

 germinal 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 

 appears to be developed, the simple columnar cells of the epi- 

 blast being directly transformed into the characteristic multi- 

 polar nerve cells. The whole of the sympathetic nervous system 



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 hypo- 

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



