As has already been mentioned, our whole understanding of an organism rests upon the cell theory established thirty years ago by Schleiden and Schwann. According to it, every organism is either a simple cell or a cell-community, a republic of closely connected cells. All the forms and vital phenomena of every organism are the collective result of the forms and vital phenomena of all the single cells of which it is composed. By the recent progress of the cell theory it has become necessary to give the elementary organisms, that is, the “organic” individuals of the first order, which are usually designated as _cells_, the more general and more suitable name of _form-units_, or _plastids_. Among these form-units we distinguish two main groups, namely, the cytods and the genuine cells. The _cytods_ are, like the Monera, pieces of plasma without a kernel (p. 186, Fig. 1). _Cells_, on the other hand, are pieces of plasma containing a kernel or nucleus (p. 188, Fig. 2). Each of these two main groups of plastids is again divided into two subordinate groups, according as they possess or do not possess an external covering (skin, shell, or membrane). We may accordingly distinguish the following four grades or species of plastids, namely: 1. _Simple cytods_ (p. 186, Fig. 1 _A_); 2. _Encased cytods_; 3. _Simple cells_ (p. 188, Fig. 2 _B_); 4. _Encased cells_ (p. 188, Fig. 2 _A_). (Gen. Morph. i. 269-289.) Concerning the relation of these four forms of plastids to spontaneous generation, the following is the most probable:—1. The _simple cytods_ (Gymnocytoda), naked particles of plasma without kernel, like the still living Monera, are the only plastids which directly come into existence by spontaneous generation. 2. The _enclosed cytods_ (Lepocytoda), particles of plasma without kernel, which are surrounded by a covering (membrane or shell), arose out of the simple cytods either by the condensation of the outer layers of plasma or by the secretion of a covering. 3. The _simple cells_ (Gymnocyta), or naked cells, particles of plasma with kernel, but without covering, arose out of the simple cytods by the condensation of the innermost particles of plasma into a kernel, or nucleus, by differentiation of a central kernel and peripheral cell-substance. 4. The _enclosed cells_ (Lepocyta), or testaceous cells, particles of plasma with kernel and an outer covering (membrane or shell), arose either out of the enclosed cytods by the formation of a kernel, or out of the simple cells by the formation of a membrane. All the other forms of form-units, or plastids, met with, besides these, have only subsequently arisen out of these four fundamental forms by natural selection, by descent with adaptation, by differentiation and transformation.