596 



CHAPTER XLIL 



ON THE DEVELOPMENT OF THE DIFFERENT ORGANS. DEVELOPMENT 



OF THE SPINAL COLUMN. OF THE FACE AND VISCERAL ARCHES. 



DEVELOPMENT OF THE NERVOUS SYSTEM. OF ORGANS OF VISION 



AND HEARING. - DEVELOPMENT OF THE HEART AND AORTIC 



ARCHES. OF THE ANTERIOR VENOUS TRUNKS. OF THE LUNGS. 



OF THE THYROID. DEVELOPMENT OF THE ALIMENTARY CANAL, 



OF THE LIVER AND PANCREAS. OF THE SPLEEN. DEVELOP- 

 MENT OF THE WOLFFIAN BODIES AND KIDNEYS. OF THE SUPRA- 

 RENAL CAPSULES. OF THE ORGANS OF GENERATION. 



IN the present chapter we have to consider the mode of deve- 

 lopment of the most important organs of the body ; but we do not 

 propose to enter further into the process of development of the 

 separate tissues, as this part of the subject has been already treated 

 of in the preceding chapters upon the anatomy of the different 

 organs. 



Development of the Spinal Column. In man and the higher ver- 

 tebrata, the spinal column is composed of a number of distinct and 

 separate segments, which are connected together by the interven- 

 tion of a fibrous material. This gives to the whole column a con- 

 siderable amount of mobility. In the lower fishes, however, no 

 such division exists; and in the place of numerous vertebras we 

 have a continuous mass of a soft consistence running through the 

 whole length of the animal, and known as the Chorda Dorsalis. 

 The material of which this is composed is the simplest form of 

 cartilage, consisting entirely of a number of large cells, without 

 the interposition of any matrix or intercellular material between 

 them. In the embryonic condition of all vertebrate animals, we 

 meet with a chorda dorsalis entirely composed of cells, and possess- 

 ing similar characters to the permanent chorda dorsalis of the 

 cartilaginous fishes. It is seen as a faint streak at the bottom 

 the primitive groove. Above it, the central organs of the nervous 



