THE DEVELOPMENT OF AUMETQS PUMILIO. 607 



different directions, the rounder nuclei of the outer layer, 

 which measure '01 mm. in length and '008 mm. in breadth, 

 being- mostly inclined at an angle of about 75° to the surface ; 

 the more oblong nuclei of the inner layer, measuring '01 mm. 

 in length and '004 mm. in breadth, lie parallel to the outer 

 surface of the embryo. This inner layer is continued for- 

 wards, and forms a skin covering the inner surface of the 

 brain ; it is of mesodermatic origin. 



The first step towards the differentiation of the brain out 

 of the epidermis just described consists of a rapid thickening 

 of the outer or germ layer, through the multiplication of its 

 cells. It soon becomes six to seven cells thick, and the nuclei 

 have no longer any common direction. Through the thicken- 

 ing of the outer layer the surface of the embryo has here 

 become a little arched. After this the outer or germ layer 

 again becomes thinner, and only consists of one layer of cells. 

 The inner layer also thickens till it becomes two or three cells 

 deep; but as it does not take part in the construction of the 

 brain I shall refer to it again as little as possible. 



After proceeding forwards a short distance, probably 

 corresponding to one abdominal segment, the cell-mass of the 

 germ layer becomes again reduced to a single layer of cells ; 

 then it thickens again as before. 



When the germ layer has reached its former thickness we 

 observe several important changes. In the first place a new 

 element has begun to develop. The cells of the germ-la3^er 

 nearest to the surface have changed the direction of the axes 

 of their nuclei. The nuclei themselves have become much 

 elongated, measuring now •015 mm. in length by '004 mm. in 

 breadth ; they have also become poorer in chromatin. In this 

 manner a new outer integument, the definite epidermis, has 

 been formed. 



The cells forming the germ-layer were till now equally 

 distributed throughout the whole of the layer; this now 

 ceases to be the case. A little space void of nuclei is foi-raed 

 on the surface of the germ-layer. This is filled with plasm; 

 just above it there is a gap in the epidermis. For convenience, 



