330 



THE CAT. 



[CHAP. x. 



This ultimate diversity is amply sufficient to show that a real differ- 

 ence existed from the first a difference thus demonstrable to our 

 reason, though not manifest to our senses. 



From the primitive substance of which the cat's ovum consists, all 

 the ultimate constituent parts of its body are derived through the 

 help of the ceUformations t already described as the epiblast, hypoblast, 

 and mesoblast. It has already been mentioned that the epiblast gives 

 rise to the epidermis of the skin and to the nervous centres ; the 

 hypoblast, to the alimentary epithelium ; and the mesoblast, to the great 

 mass of the body. But parts which are derived from one of these 

 sources may acquire characters quite like those derived from another. 

 Thus the linings of the two ends of the alimentary canal are (as has 

 been said) formed from inflected epiblast, and epithelial structures 

 (as in the lining of the vessels and of the peritoneal cavity) can be 

 formed as well from the mesoblast as from the epiblast. 



All the tissues and organs of the cat's body are then derived 

 from cells, and indeed they are doubly so derived, since the ovum 

 before yelk segmentation begins, is a perfect cell with its cell-icall 

 (or periplast] and its nucleus (or cndoplast), the latter being fur- 

 nished with one or more nucleoli. Thus this cell begets the cells 

 of the three layers of the embryo, and these latter cells beget all the 

 tissues and organs which subsequently arise, and the great mass of 

 them are begotten by the cells of the mesoblast. 



But though all the tissues have this ultimate cellular parentage, 

 they by no means always retain a plainly cellular structure, as they 

 severally arise from the primitive, or "indifferent" tissue of the 

 mesoblast, and become definite connective, muscular or nervous 

 substance, as the case may be. Sometimes they take on the form 

 simply of a soft substance of one or another kind, within which 

 nuclei are embedded at intervals. From analogy we may regard 

 the parts of such substance which are adjacent to such nuclei as 

 representing cells, the limits of which are severally indistinguishable. 



The five main constituents of the cat's body (1) connective tissue, 

 with its derivatives, cartilage and bone ; (2) epithelial tissue ; (3) 

 blood; (4) muscular tissue; and (5) nervous tissue arise as follows : 



Connective tissue appears to arise in the embryo, partly as a jelly- 

 like substance, or matrix, and partly as cells from the mesoblast, 

 which though more or less separated by this substance, yet remain 

 connected by processes which grow out in a radiating manner from 

 them. The fibres of the tissue are by some observers described as 

 arising within the protoplasm of the cells, those of adjacent cells unit- 

 ing, while the parts of the cell not thus transformed persist as connec- 

 tive-tissue corpuscles. Other observers, however, believe that the fibres 

 arise, as an independent deposit, within the intercellular substance. 



Elastic tissue is said to be formed from other cells which grow out and 

 branch, becoming connected with processes from neighbouring cells. 



Cartilage appears in its simplest condition (in the chorda dorsalis 

 or notochord) as a mass of closely applied, thin- walled cells. The 

 layer of the embryo from which these are derived, is (as has been 



