PEODUCTION OF CELLS. 9 



the embiyo cells in order to the production of the various textures of 

 which the animal body is composed, it will be convenient to consider 

 the manner in which the cells themselves are produced, and the nature 

 of the substance composing them. 



Production of Embryo Cells. — So far as is at present known, every 

 cell in the animal body has been derived from a previously existing 

 cell. In the case of the cells which compose the early embryo this 

 parent cell is, as has been previously pointed out, the ovum itself, or at 

 least its germinative part. 



The mammalian ovum differs indeed from the cells we have just 

 been considering, both in its size, and in possessing a stout external 

 membrane (fig. 5 a). Like them, however, it mainly consists of a 

 protoplasmic substance (b), in which are embedded fatty granules (the 

 I/elk), and contains structures (the germinal vesicle {c) and germinal 

 spot) , which are comparable respectively to the nucleus and nucleolus. 



Fis;. 5. 



Fig. 5. — Diagrammatic Figures to illustrate the Formation of Cells within the 

 I\LuiMALiAN Ovum by Segmentation of the Yelk ; magnified. 



a, external membrane ; h, protoplasmic contents ; c, germinal vesicle containing the 

 germinal spot. 



The embryonic cells are produced from the ovum by a process of 

 cleavage or segmentation, of which the following is an outline : — 



The germinal vesicle disappears ; the contents of the ovum then 

 shrink somewhat and separate into two equal parts (b) ; the first two 

 segments divide each again into two (c), and the binary division thus 

 goes on (D, E,) pretty regularly until the whole is transformed into 

 a number of small segments, the embryonic cells, each consisting, as 

 we have seen, of protoplasmic matter enclosing a nucleus. The latter 

 is not always discoverable in the earlier segments, l^eing perhaps 

 hidden by the opaque granular mass, but it soon comes into view, and 

 has been supposed to play an important part in the formation of the 

 cells. At all events it may be observed, in those segments in which 

 the nucleus is visible, that the division of this body precedes that 

 of the protoplasmic substance. Whether the first nucleus is itself 

 derived from the vanishing germinal vesicle and spot is unknown. 



