62 THE EVOLUTION OF MAN. * 



were put forward with unusual presuraption, and professed 

 to disprove Baer's Theory of Germ-layers. They are 

 written in so clumsy and confused a style, that no one 

 could quite understand them ; but for this very reason they 

 won the admiration of many readers, who supposed that 

 a nucleus of profound wisdom was hidden somewhere be- 

 hind these obscure oracular and mysterious sayings. 



Kemak was the first to throw full light on the great 

 confusion which Reichert had caused, by explaining, in the 

 simplest possible manner, the evolution of the tissues. Ac- 

 cording to him, the egg of animals is always a simple 

 cell, and the germ-layers, which proceed from the egg, are 

 also composed only of cells, and those cells, which alone 

 constitute the egg, are produced in a very simple manner 

 by the continuous and repeated segmentation or dividing 

 up of the original simple egg-cell. This cell divides, or 

 parts, first into two, and then into four; from these four 

 arise eight, then sixteen, and then thirty-two, and so on. 

 Hence, in the individual evolution of every animal, as well 

 as of every plant, from the one simple cell, constituting the 

 egg, is formed, by repeated segmentation, an aggregate of 

 cells, as Kolliker had already maintained in 1844. The 

 cells of such a mass spread themselves out flatly, and so 

 form into layers, so that every one of these layers is 

 originally composed of but one kind of cell. The cells of 

 the layers differentiate themselves, or assume various forms ; 

 and then there is a further differentiation, or, in other 

 words, a division of labour of the cells within the layers 

 themselves, and this latter differentiation produces all the 

 various tissues of the body. 



These are the very simple principles of Histogeny, or 



