8o THE ASCENT OF MAN 



building entirely new rooms, or by partitioning old 

 ones. Both of these methods are employed in 

 Nature. The first, gemmation, or budding, is com- 

 mon among the lower forms of life. The second, 

 differentiation by partition, or segmentation, is the 

 approved method among higher animals, and is that 

 adopted in the case of Man. It proceeds, after 

 the fertilized ovum has completed the complex 

 preliminaries of karyokinesis, by the division of 

 the interior-contents into two equal parts, so that 

 the original cell is now occupied by two nucleated 

 cells with the old cell-wall surrounding them out- 

 side. The two-roomed house is, in the next de- 

 velopment, and by a similar process of segmentation, 

 developed into a structure of four rooms, and this 

 into one of eight, and so on.^ In a short time the 



^When the multicellular globe, made up of countless off- 

 shoots or divisions of the original pair, has reached a certain 

 size, its centre becomes filled with a tiny lakelet of watery 

 fluid. This fluid gradually increases in quantity, and, pushing 

 the cells outward, packs them into a single layer, circum- 

 scribing it on every side as with an elastic wall. At one 

 part a dimple soon appears, which slowly deepens, until a 

 complete hollow is formed. So far does this invagination of 

 the sphere go on that the cells at the bottom of the hollow 

 touch those at the opposite side. The ovum has now become 

 an open bag or cup, such as one might make by doubling in 

 an india-rubber ball, and thus is formed the gastrula of biology. 

 The evolutional interest of this process lies in the fact that 

 probably ^U animals above the Protozoa pass through this gas- 



