- *6 - 



parts adjacent to the same, the yolk spherules have 

 become very small indeed; which would lead one to think 

 that the mesoderm is causing the yolk to break up. 



The outer membrane shows no difference from that 

 described in Stage I. The inner has, however, changed 

 since the first stage, having risen above the surface of the 

 egg in the region of the blastoderm, leaving a free space 

 between itself and the ectoderm. 



Stage III. 



The third stage at my disposal is considerably more 

 advanced than the last. The reversion of the embryo has 

 already taken place, and the extremities have begun to 

 grow. I have only found one egg at this stage; it comes 

 from the same batch as the eggs described unter Stage I 

 and Stage IV. Its growth had evidently been stopped by 

 the pressure of the surrounding eggs, which also caused 

 the embryo to develop in two disconnected halves, only 

 held together by the yolk. The embryo also showed 

 several other signs of not having developed normally. 



My sections through this egg are, unfortunately, not 

 all that I could wish, but still they are good enough to 

 make out several details. For this reason I shall only 

 attempt to describe a few of the organs. 



The brain is already divided into its three parts, the 

 commissures 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, and 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 rever- 

 sion 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. 



