8 ZOOLOGY. 
still firmer intercellular substance ; and when the intercel- 
lular substance becomes combined with salts of lime form- 
ing bone, we have bony tisswe. 
The blood-corpuscles originate from the mesoderm as 
independent cells floating in the circulating fluid, the blood- 
cells being formed contemporaneously with the walls of the 
vessels enclosing the blood. In the invertebrates the blood- 
cells are either strikingly like the Ame@da in appearance, or 
are oval, but still capable of 
changing their form. Thus blood- 
é ; corpuscles arise like other tissues, 
of awaier beetle” After Minot ot that they finally become 
ree. 
Muscular tissue is also composed of cells, which are at 
first nucleated and afterward lose their nuclei. From being 
at first oval, the cells finally become elongated and more or 
less spindle-shaped, forming fibres; these unite into bundles 
forming muscles. ach fibre is ensheathed in a membrane 
called sarcolemma. Muscular fibres may be simple or striated 
(Fig. 5). The contractility of muscles is due to the con- 
tractility of the protoplasm : 
originating in the cells forming 
the fibres. 
Nervous tissue is made up 
ef nerve-cells and fibres pro- 
ceeding from them; the for- 
mer constituting the centres 
of nervous force, and usually 
massed together, forming a 
ganglion or nerve-centre from 
which nerve-fibres pass to the Fig. 6.—A gunglion in the clam, with 
periphery and extremities of ®°Ye (é, g, &) proceeding from it, 
the body, and serve as conductors of nerve-force (Fig. 6). 
Organs and their Functions.—Having considered the 
different kinds of cells and the tissues ‘they form, we may 
now consider the origin of organs and their functions. The 
Protamoba may be considered as an organless being. In 
Ameba (Fig. 11) we first meet with a specialized portion of 
the body, set apart for the performance of a special function 
