DEVELOPMENT OF THE EGG. 175 



forces, and have like powers of resistance to the morbific 

 element. , 



The more highly organized the structure the longer it seems 

 to take for arriving at full maturity. Thus puberty, brain- 

 cells, etc., illustrate. The process through which maturity 

 arrives is through a series of constant changes and constant 

 advancement. We all recognize this element of time in the 

 growth and changes of function in the young ; it is also con- 

 clusively illustrated by the well-established law of inheritance, 

 through which there is a tendency to inherit at a correspond- 

 ing age. 



There also seems to be an antagonism between the processes 

 of growth and development, as has been before remarked on. 

 This is but an expression of the fact that forces utilized in 

 one direction cannot, at the same time, be utilized for another 

 purpose. 



If such be the genesis of the progress of cells towards 

 higher capabilities, the same laws must hold true for the com- 

 pleted life — the animal — the harmonious aggregation of these 

 unities and their results. 



Going back to the egg, we will trace somewhat of the 

 development, as illustrated by the mammalian ovum. The 

 organized ovum receives additional force through the conjunc- 

 tion with the spermatozoa, and is now enabled to proceed 

 with changes in accordance with its capabilities. 



The egg consists of a membranous external sac or envelope, 

 inclosing the spherical yolk, within which is the germinal 

 vesicle and its nucleus. The nucleus, or the germiual spot, is, 

 as we have heretofore said, the spot where is the life, or the 

 power of originating the life. The yolk, however, has a most 

 essential share in the development of the embryo. It may be 

 considered as the contents of a cell of which the germinal 

 vesicle is the nucleus, if we regard this more completed stage 

 of development as a single cell, as it is claimed to be by 

 Schwann. 



The fertilized — that is, strengthened — ovum in a short 

 period commences to show changes. According to Barry, 

 the immediate effect of fecundation is to cause the germinal 

 spot to pass to the centre of the vesicle and the vesicle to the 

 centre of the yolk. The spot first indicates a change, which 



