THE DEVELOPMENT OF ADMETTJS I'UMILIO. 601 



The brain is already divided into its three parts^ the com- 

 missures and the two kinds of nuclear tissue, the difference 

 of which will be explained in full in Stage IV. These two 

 tissues are chiefly characterised by the size, tinction, and 

 density of the nuclei they contain, the smaller being 

 closely packed, aud stained darker. They measure '005 mm., 

 the larger, lighter stained, and less dense nuclei, on the other 

 hand, measuring as much as '009 mm. to "01 mm. 



The ventral ganglion-cord has not yet begun to appear. 

 Those parts of the abdomen and cephalothorax which are 

 bent upon each other at the time of the reversion of the 

 embryo are devoid of any cellular integument, although 

 lined by a thick cuticle. It is in this part that the ventral 

 ganglion- cord takes its origin in the next stage. 



It is interesting to observe that, as in spiders, the legs are 

 full of yolk, only the tips being free. The yolk seems to get 

 pushed backwards during the growth of the legs pari passu 

 with the growth of the muscles and nerves. Neither the 

 median nor the lateral eyes have yet begun to appear, nor 

 are the lungs, heart, coxal gland, lateral organs, or alimentary 

 canal in any degree traceable. 



It is in the yolk that the chief peculiarity is to be seen. 

 The neighbouring eggs have, through pressure, caused a 

 deep semicircular impression on one side of the embryo, and 

 at the same time caused it to develop in two halves. The 

 pressure has also influenced the yolk, causing several of the 

 yolk-spherules to amalgamate and to form a mass without 

 any regular shape. The appearance of this fusion necessi- 

 tates the conclusion that the living yolk particles were of a 

 fatty nature, and that they were suspended in a watery lymph, 

 forming a kind of emulsion. 



Stage IV. 



The next stage which I possess is only a little more 

 mature than the one just described. The reversion of the 

 embryo has taken place long ago, the cephalothorax being 



