48 DEVELOPMENT OF THE TISSUES FROM CELLS. [CHAP. I. 



by areolar tissue and blood-vessels, which are foreign to the true 

 adipose cells ; and, in the grey nervous matter, vessels and nerve 

 tubes exist between the cells. 



The sclerous tissue (cr/c\,r)pos, hard,) contains a large proportion 

 of inorganic material, to which it owes its hardness ; it differs very 

 materially from all the other tissues, excepting cartilage and fibro- 

 cartilage, which, as regards hardness, might be classed with it. 



The compound tissues are those, the elementary parts of which 

 are made up of two distinct tissues. Thus both muscle and nerve 

 are composed of parallel fibres or threads, each fibre being com- 

 pound ; in muscle, it is composed of homogeneous membrane, dis- 

 posed like a tube, containing a fleshy (sarcous) substance, arranged 

 in a particular manner, which is the seat of the vital properties of 

 the tissue ; and, in nerve, the fibres are composed of similar tubes 

 of homogeneous membrane containing an oleo-albuminous sub- 

 stance, neurine. Fibro-cartilage is also properly a compound tex- 

 ture, being made up of white fibrous tissue and cartilage ; it is 

 employed almost exclusively in the mechanism of the joints of 

 the skeleton, in which it is associated with bone, cartilage, and 

 ligaments. 



Of the Development of the Tissues from Cells. At the earliest period 

 of embryonic life, the process of organization has advanced to so 

 slight an extent, that the variety of textures above described has 

 not yet appeared. 



The prevailing mode, in which the development of animals takes 

 place is, by the formation, within the parent, of a body containing 

 the rudiments of the future being, as well as a store of nutrient 

 material sufficient to nourish the embryo for a longer or shorter 

 period. This body is called the ovum or egg. It is of that form 

 which, in a former page (p. 9, fig. 1), has been described and 

 delineated as the simplest which organization produces. It con- 

 sists of a vesicular body filled by a fluid, and enclosing another, 

 within which is a third, consisting of one or more minute, but 

 clear and distinct granules. The first, or the vitelline membrane 

 of the ovum, is the wall of a cell ; it is composed of homogeneous 

 membrane : the second, or the germinal vesicle of the egg, is the 

 nucleus of the first : and the third, which is called by embryologists 

 the germinal spot, is a nucleolus to the second. It appears, from 

 the researches of Wagner and Barry, that the nucleus or germinal 

 vesicle precedes the formation of the vitelline membrane, but the 

 precise relation as to the period of its formation of the nucleolus 

 or germinal spot to the nucleus has not yet been satisfactorily 



