84 MAN AND THE LOWER ANIMALS. n 



— within which is enclosed a second much more 

 delicate spheroidal bag, called the germinal vesicle 

 (a). In this, lastly, lies a more solid rounded body, 

 termed the germinal spot (b). 



The egg, or Ovum, is originally formed within 

 a gland, from which, in due season, it becomes 

 detached, and passes into the living chamber fitted 

 for its protection and maintenance during the pro- 

 tracted process of gestation. Here, when sub- 

 jected to the required conditions, this minute and 

 apparently insignificant particle of living matter 

 becomes animated by a new and mysterious activ- 

 ity. The germinal vesicle and spot cease to be 

 discernible (their precise fate being one of the yet 

 unsolved problems of embryology), but the yelk 

 becomes circumferentially indented, as if an in- 

 visible knife had been drawn round it, and thus 

 appears divided into two hemispheres (Fig. 

 13, C). 



By the repetition of this process in various 

 planes, these hemispheres become subdivided, so 

 that four segments are produced (D); and these, 

 in like manner, divide and subdivide again, until 

 the whole yelk is converted into a mass of 

 granules, each of which consists of a minute 

 spheroid of yelk-substance, inclosing a central 

 particle, the so-called nucleus (F). Nature, by 

 this process, has attained much the same result 

 as that which a human artificer arrives at by his 

 operations in a brick-field. She takes the rough 



