16 INTRODUCTION. 
for a time is bound to the pericardial walls by dorsal and ventral mesocardia ; 
there is, in mammals, a mediastinum between the two pleural cavities, connect- 
ing the pericardium to the body wall, while frequently the ovaries and the testes 
project into the ccelom, carrying the peritoneum with them, thus giving rise 
to a mesOvarium or a mesorchium, according to the sex. 
The Mesenchyme has two chief places of origin. One is from 
the splanchnic wall of the segments of the mesomere, each of which 
is the centre of rapid cell proliferation and forms the sclerotome (fig. 
8, st), since some cells arising from it are concerned in the formation 
of the axial skeleton. These cells pass in to surround the notochord, 
and upward on either side of the central nervous system and downward 
beside the alimentary canal, thus forming a partition between the 
two sides of the body. A second source of the mesenchymatous cells 
is from the somatic wall of each myotome, all of the cells of which are 
transformed into this layer, and lie immediately beneath the ectoderm. 
Thus there is a complete envelope of mesenchyme around the whole 
body. From these and from other sources the mesenchyme extends 
everywhere in the remains of the segmentation cavity—between the 
muscles and around the various viscera—forming a framework in 
which the products of all the other layers are enveloped (fig. 30). This 
mesenchymatous framework has great importance in the development 
of the skeleton and its general plan will be described in connection with 
the skeletal structures. 
HISTOLOGY. 
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 connected mass of 
cells similar in appearance and function, together with a varying 
amount of intercellular substance, usually formed by the cells them- 
selves. The study of the minute structure of animals and especially 
of the tissues is the province of histology. 
There are many kinds of tissues, only a few of which need mention 
here, but all may be grouped under four great heads: epithelial 
