HISTOLOGY 21 



From these sources the mesenchyme extends everywhere in the 

 remains of the segmentation cavity, surrounding the other layers and 

 intruding between the muscles and around the various viscera, 

 forming a framework in which the products of all the other layers are 

 enveloped (fig. ss). This mesenchymatous framework has great 

 importance in the development of the skeleton and its general plan 

 will be described in connexion with the skeletal structures. 



HISTOLOGY 0^ 



In the gastrula the cells differ from each other chiefly in position, 

 and the same is true, even when the germ layers are first differentiated. 

 As development goes on the differences between the various groups of 

 cells increase, each group becoming more specialized for some one 

 purpose and losing the power to do more than the one kind of work. 

 For community of work cells of the same kind become associated to- 

 gether, the result being tissues. A tissue, then, is a c onnec ted mass 

 of cells similar in a ppearance and fu nctio n, together with a varying 

 amount of intercellular substance, usually formed by the cells them- 

 selves. The. study qi the minute structure ofjinimals aad especially 

 of tlie tissues is_the province of hi stology. 



There are many kinds of tissues, only a few of which need mention 

 here, but all may be grouped under four great heads: epithelial, 

 nervous, muscular and connective tissues; the members of each group 

 having certain fundamental points in common. 



Epithelial Tissues 

 Epithelia are_the covering tissues, and_ occur on any free surfac e,. 



internal or external, of the b ody. Both comparative anatomy and 

 embryology show theni to be the primitive tissues, for there are many 

 lower animals which are made up entirely of epithelia, while in the 

 vertebrates the embryo consists solely of epithelia until the mesen- 

 chyme appears. Epithelia may come from any of the germ layers, 

 in rare cases (synovial cavities) even from mesenchyme. 



The character of epithelium varies according to the nature of 

 the work it has to perform. That on the outside of the body is largely 

 protective, hence it is often thickened and strengthened in different 

 ways to afford resistance against external injuries. In other places, 

 as glands, it has to elaborate and to allow the passage outward of 

 material from within. In the body cavity and in the blood-vessels 



