1 86 THE EVOLUTION OF MAW. 



vvliich will be explained later, are called Cleavage-cells or 

 Oleavage-globules (segmentella). The repeated process of 

 division of the parent-cell, which gives rise to the cleavage- 

 cells, has long been known as egg-cleavage, or, inaccurately, 

 as cleavage (segmentation). At an earlier or later stage, the 

 entire mass of cleavage-cells divides into two essentially 

 different groups, which range themselves in two separated 

 cell-strata ; the two primary germ-layers. This formation of 

 the germ-layers is a process of the greatest significance, and 

 the real beginning of the formation of the true animal body. 



It is only quite recently that the fundamental germinai 

 processes of egg-cleavage and the formation of the germ- 

 layers have been thoroughly understood, and their real 

 significance rightly estimated. In the various animal groups 

 these processes exhibit various striking differences, and it 

 was no easy task to show their essential similarity or 

 identity throughout the whole animal kingdom (always 

 excepting, of course, the Primaeval Animals, or Protozoa). 

 It was only after I had established the Gastraea Theory,* 

 in 1872, and afterwards, in 1875, had traced back indi- 

 vidual forms of egg- cleavage and of the formation of the 

 gastrula to one and the same type-form, that this important 

 identity could be regarded as really proved. This furnished 

 a single law which conditions the earliest germinal processes 

 of all animals. 66 



The relation of Man to these earliest and most import- 

 ant processes is entirely similar to that of other higher 

 Mammals, and especially to that of Apes. As the human 

 germ or embryo, even in a much later stage of its formation, 

 when the brain-bladders, the eyes, the organs of hearing, 

 the gill-arches, etc. are also present, does not essentially 



