THE CELL OR ELEMENTARY PART. 47 



matter left, that it may be easily be overlooked. In disease, 

 on the other hand, the germinal matter may increase to three 

 or four times its ordinary dimensions, when it becomes a 

 very striking object. The ovum at an early period of its 

 development is but a cell consisting of a mass of germinal 

 matter, with a new centre and often with numerous new 

 centres (known as germinal spots or nuclei) embedded in it, 

 enclosed in a capsule of formed material. 



The mode of formation of the cell, or elemental unit, 

 as well as the origin from it of other units is well illustrated 

 in the formation of the ovum. In pi. Ill, fig 7, the cells 

 constituting the tissue of the ovary of the common stickle- 

 back are represented, and amongst them are seen true ova 

 at a very early period of development. The youngest of 

 these differs but little from the cells amongst which it lies. 

 It is, in fact, but one of these which has advanced in de- 

 velopment beyond the rest. In fig. 8, a small but complete 

 ovum is seen with its germinal, or living matter, here called 

 germinal vesicle, surrounded by the yolk which consists of 

 formed matter. In the germinal matter are seen numerous 

 germinal spots, which are new living centres of growth 

 originating in living matter. In these are new centres, figs. 

 9, 10, n, and in these last others would have appeared at a 

 later period. In all cases the lifeless nutrient material must 

 pass into the very centre of the living particles, before the 

 peculiar vital properties are communicated to it. 





