IS8 THE EVOLUTION OF MAN. 



This original, palingenetic form of egg-cleavage, and 

 of the formation of the germ-layers is altogether unrepre- 

 sented in the present day in the Vertebrate tribe, to which 

 !Man belongs, except in the lowest and oldest member of 

 this tribe, the remarkable Lancelet or Amphioxus (Cf. 

 Chapters XIII. and XIV., and Plates X. and XL). But it 

 is still found in exactly this form in many low inverte- 

 brate animals — for example, in the remarkable Sea-squirts 

 (Ascidia), in the Pond-snail {lAnmcGus), in the Arrow- worm 

 (Sagitta) ; also in many Star-animals (Eckinodernia) and 

 Plant-animals, — for example, in the common Star-fish and 

 Sea-urchin, in many Medusae and Corals, and in the 

 simplest Chalk Sponges {Olynthus). As an example, let us 

 examine the palingenetic egg-cleavage and formation of 

 the germ-layers of an eight-rayed single Coral, which I 

 found in the Red Sea, and described in my Arahischen 

 Korallen under the name of Monoxenia Darwinii.^^ 



After the Monerula (Fig. 22, A) has changed into the 

 parent-cell, or cytula (jB), the latter divides into two similar 

 cells ((7). The kernel of the parent-cell first parts into 

 two similar halves ; these part asunder, shrink from each 

 other, and then act as centres of attraction to tlie surround- 

 ing protoplasm ; after this the protoplasm becomes con- 

 tracted by a circular groove running round its circumference, 

 and then separates into two similar halves. Each of the 

 two cleavage-cells, which are thus produced, again separates 

 in the same way into two similar cells, the plane of division 

 between these two latter lying at right angles to that 

 between the two former (Fig. 22, D). The four similar 

 cleavage-cells, the descendants in the second generation of 

 the parent-cell, lie in one plane. Each of these now again 



