40) THE OVUM AND 
outside of them. The generation of the cells takes place in a 
fluid, or in a structureless substance in both cases. We will 
name this substance in which the cells are formed, cell-germi- 
nating material (Zellenkeimstoff), or cytoblastema. It may 
be figuratively, but only figuratively, compared to the mother-lye 
from which crystals are deposited. 
We shall refer to this point at greater length hereafter, and 
only anticipate our subject with this result of the investigation, 
in order to facilitate the comprehension of what follows. 
In the previous section of this work we have discussed in 
detail the course of development of some of the animal cells, 
having taken the chorda dorsalis and cartilage for our examples. 
We are now required to prove, as far as is possible, that all 
the tissues either originate from, or consist of cells. We 
separate this investigation into two divisions. The first treats 
of the Ovum and Germinal membrane, in so far as they form 
the common basis of all the subsequent tissues. The second 
division embraces the permanent tissues of the animal body, 
with the omission of the two already described. 
FIRST DIVISION. 
On the Ovum and Germinal Membrane. 
The ovum of Mammalia lies, as is known, within the Graafian 
vesicle. J have not made any investigation as to whether that 
vesicle may be considered to have the signification of a cell. 
It is deed a cell in the general sense of the word, being a 
cavity in the substance of the ovary, it has even a special 
membrane ; but as we here only receive the word cell as sig- 
nifying an elementary part of animals and plants, it becomes 
necessary to inquire whether this membrane may not be a 
secondary formation resulting from the junction of other struc- 
tures which are elementary. The history of the development 
of the Graafian vesicle must show whether that be the case, or 
whether it originate by the mere growth of a cell furnished 
with a structureless cell-membrane, which cell may formerly, 
