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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 segme it, 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 thick- 

 ness we observe several important changes. In the first 

 place a new element has begun to develop. The cells of 

 the germ- layer nearest to the surface have changed the 

 direction of the axes of their nuclei. The nuclei them- 

 selves 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 formed on the surface of the germ-layer. This 

 is filled with plasm; just above it there is a gap in the 

 epidermis. For convenience sake I shall call such super- 

 ficial masses of un-nucleated protoplasm „surface pits"; 

 this name I give only with reference to their 

 appearance and not, to their structure, and I 

 desire that this may be distinctly borne in 

 mind. The nuclei of the cells of the germ-layer 

 adjacent to this "surface pit" are beginning to 

 arrange themselves radially around the centre of the 

 hollow. Further on, again proceeding forwards, the 

 germ-layer again presents its former appearance. Still 

 proceeding towards the cephalothorax, we meet with 

 another new change. Just before the next thickening 

 containing a "surface pit" is reached, the mesodermatic 

 inner skin begins to lift itself from off the nuclei of the 

 germ-layer. The space between botn is now filled with 

 nerve-fibres, the longitudinal fibres being the first to 

 appear, as in Scorpions (Brauer, [7]). This "surface pit" 



