THE ASCENT OF THE BODY. 63 



mentation, 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 outside. The 

 two-roomed house is, in the next development, and by 

 a similar process of segmentation, developed into a 

 structure of four rooms, and this into one of eight, and 

 so on. 1 In a short time the number of chambers is so 



1 When the multicellular globe, made up of countless offshoots 

 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 out- 

 ward, packs them into a single layer, circumscribing 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 yastrula 

 of biology. The evolutional interest of this process lies in the 

 fact that probably all animals above the Protozoa pass through 

 this gastrula stage. That some of the lower Metazoa, indeed, 

 never develop much beyond it, a glance at the structure of the 

 humbler Coelentcrates will show the simplest of all illustra- 

 tions of the fact that embryonic forms of higher animals are often 

 permanently represented by the adult forms of lower. The chief 

 thing however to mark here is the doubling-in of the ovum to 

 gain a double instead of a single wall of cells. For these two 

 different layers, the ectoderm and the endoderm, or the animal 

 layer and the vegetal layer, play a unique part in the after- 

 history. All the organs of movement and sensation spring from 

 the one, all the organs of nutrition and reproduction develop from 

 the other. 



