GENERAL PRINCIPLES OF ZOOLOGY. 



appearance it belongs to the surface of the body, because 

 this mediates the relations with the external world, the 

 regulation of which relations is subserved by the central 



mf mf 



- mp 



1. 



~ep 



II. 



III. 



FlG. o. Cross-sections through'the Dorsal Regions of Triton embryos at three different ages.. 

 (From O. Hertwig.) I. The Medullary Plate (rudiment of the spinal cord), nip. is; 

 marked off from the skin, </, by the medullary folds mf. II. The Medullary Plate has; 

 become bent into a furrow through apposition of the medullary folds. III. The Medul-- 

 larv Plate has closed up, forming the spinal cord. ?/(/", medullary folds ; >n/>. medullary 

 plate ; n, the medullary or neural tube (spinal cord) arising from it ; <?/, skin (epider- 

 mis) ; c/i, notochord ; ik, middle germ-layer ; ttnA- 1 , its parietal, mk"*, its visceral layer); 

 c, coelom (body-cavity); cf, cavity of a primitive segment or mesoblastic somite ; ik,. 

 inner germ-layer ; dz, its yolk-cells ; Ig, lumen of the gut. 



nervous system. In the case of higher organized animals, 

 e.g. the vertebrates, the brain and spinal cord lie deeply 

 embedded in the interior of the body ; but in the embryo* 



