FOUR ORDERS OF PLASTIDS. 347 
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 
simyple 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 forma- 
tion 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. 
By this theory of plastids, by deducing all the different 
forms of plastids, and hence, also, all organisms composed 
of them, from the Monera, we obtain a simple and natural 
connection in the whole series of the development of nature. 
The origin of the first Monera by spontaneous generation 
