NUCLEUS AND NUCLEOLUS. 1 33 



Ontogeny can be demonstrated in answer to the problem : 

 " How does a many-celled organism arise from a one-celled 

 organism ? " Every individual organism is originally a 

 simple cell, and as such, an elementary organism, or an 

 individual of the first order. It is only at a later period 

 that this cell gives rise, by division, to a multitude of cells, 

 from which the many-celled organism, an individual of a 

 higher order, is developed. 



If we next observe somewhat more closely the original 

 composition of the egg-cell, we notice the very remark- 

 able fact, that in its original condition it is so exactly 

 the same in Man as in all other animals, that it is im- 

 possible to discover any essential difference. At a later 

 period, the eggs, even when they remain one -celled, are 

 very different in size and form, have different coverings, etc. 

 But, if they are sought in the place where they originate, 

 in the ovary of the female animal, these primitive eggs, in 

 the first stages of their life, are found to be always of the 

 same formation — every primitive egg being originally an 

 entirely simple, somewhat round, moving, naked cell, pos- 

 sessing no membrane, and consisting only of the nucleus 

 and protoplasm (Fig. 10). These two parts have long 

 borne distinctive names; the protoplasm being called the 

 vitellus, or yelk, and the nucleus the germinal vesicle, 

 (vesicula gernninativa). As a rule, the nucleus of the egg 

 is of a soft, often vesicular texture. Within this nucleus, 

 as in many other cells, is enclosed a third body, which in 

 ordinary cells is called the nucleolus. In the egg-cell it is 

 called the germinal spot {piacula gevminativa). Lastly, in 

 many, but not in all eggs, within this germinal spot, is found 

 yet another little point, a nucleolinus, which may be called 



