34. _Cypris tristriata._ 35. _Cypris tristriata_, eggs of; 35 _a_, _b_, _c_, the same hatching. 36. _Acarus domesticus_ (Cheese-mite), female. 36*. Cilia of gills of Oyster. 37. _Trombidium fuliginosum_: _a_, pulp; _b_, mandible; _c_, foot; _d_, natural size; _e_, hair; _f_, hair of _T. holosericeum_. 38. _Acarus domesticus_, male. 39. _Membranipora pilosa._ [Illustration: Plate IX. W Bagg sculp _London: John Van Voorst._] CHAPTER X. ANIMAL ELEMENTS AND TISSUES. The tissues of which animals consist, like those of plants, are primarily derived from cells; in fact the essential part of the egg or óvum, from which all perfect animals originate consists at first only of a simple cell, with its nucleus and nucleolus. The animal cell-wall differs from that of the vegetable cell in its softness and delicacy--also in its chemical composition,--the former consisting of albúminous (_albúmen_, white of egg) matter, while the latter is composed of cellular or vegetable-cell substance. There is also a striking difference between vegetable and animal tissues, in the circumstance that, while the former retain their cellular condition to a very great extent, the cells of the latter are frequently so altered by compression and fusion together, or are obscured by the great development of the cell-contents, that the cell-form is obliterated, or can only be discovered by the application of chemical reagents; and in many instances, the relation of the tissues to the cell can only be discovered by tracing the growth or development of the latter from its earliest stages. Hence the examination of the elements and tissues of animals is not well adapted for those who are unpractised in the use of the microscope; and in treating of them, we shall simply notice a few which are most easily examined, beginning with those found in animals belonging to the subkingdom Vertebráta (_ver´tebra_, a spine-bone). MAMMÁLIA.--The animals belonging to this class suckle their young; and their blood-vessels contain red blood.