BLOOD-CORPUSCLES.
never distinctly observed nucleoli in it; occasionally only I thought I perceived something of the kind, for instance, in the blood-corpuscles of a salamander ; it was not, however, sufciently evident to permit of my asserting their presence. Cell-contents must certainly exist; for if the cell-walls lay immediately upon one another, the corpuscle must be as much thinner on the margins beside the nucleus as the thickness of the nucleus amounts to. If it be assumed that the cell-membrane alongside the nucleus may be so much thicker as thereby to produce the almost level side surfaces, the cell-membrane must in such case have a thickness equal to the half of that of the corpuscle; but it would then be sufficiently thick to allow of a double outline being distinguished when it was swollen by water ; observation, however, does not detect any such appearance. The red colouring matter forms the cell-contents. It is difficult to decide whether the cell-membrane and nucleus are also coloured, but it is in some degree probable that they are so, since otherwise the centre of the corpuscle where the nucleus lies must appear white, whilst it in fact exhibits a paler red colour. The colouring matter of the blood-cells is not contained in granules, as it is in most kinds of pigment, but in a state of solution. If the lymph-corpuscles be cells, their transformation into the blood-corpus- cles may at least be conjectured as taking place by their becoming flattened and absorbing colouring matter. Those blood-corpuscles in which the envelope (hülle) is smaller in proportion to the nucleus, a fact often observed in the frog, are probably younger cells. I have made no observations upon the formation of the blood-corpuscles in the germinal membrane. According to C. H. Schultz (System der Cirkulation, p. 33), the blood-corpuscles in the chick are formed round the yelk-globules. (?) The latter are first present, and form the nucleus of the blood-corpuscles; they become surrounded with a delicate membrane. The vesicle then dilates, and at length becomes flattened. This description accords excellently with the fundamental laws previously developed, and shows that as early as 1836 Schultz had discovered the pre-existence of the nucleus of the blood-corpuscle, the formation of the blood-vesicle around it, and the gradual expansion of that vesicle.
