— 15 - 



The cause of the group lying deeper than the last 

 "described is an ingrowth of the epidermis in the shape 

 of a narrow tube. Its bore is *007 mm., and its length 

 •01 mm. The wails of this tube are formed by a single 

 layer of cells, whose nuclei are but lightly stained, and 

 which contain very large micro-nuclei. One nucleus 

 1 measured was *01 mm. long by # 004 mm., its micro- 

 nucleus, however, actually measuring *004 by *003 mm. 



The nuclei of the cells forming this tube are all 

 arranged with their long axes lying at right angles to the 

 axis of the tube. The tube ends in an expansion, forming 

 a nearly spherical sac, whose diameter is about '01 mm. 

 The walls of this sac are formed by the ends of the radial 

 supporting fibres. The sac just described lies within the 

 funnel-shaped sheath, which encloses the radial supporting 

 fibres. The next group of cells so much resembles the 

 last described that it is not necessary to go into any 

 details about it. 



Still proceeding forwards, we now meet with a group, 

 which half retains its original form and independence, 

 and is half connected with and converted to the ventral 

 ganglion-cord in its definite form. The original germ-group 

 lies *1 mm. from the surface; its inner surface abuts on 

 the commissures. On the side of the group which is turned 

 towards the abdomen, the sheath covering the radial 

 supporting-fibres and the germ cells forming the group are 

 as before, but the cells forming the tube have multiplied 

 rapidly, and now reach as far as the cells forming the 

 group. On the anterior side all parts of the group have 

 multiplied their cells to such an extent that they build a 

 solid mass of nuclei, reaching from the surface to the 

 commissure. In this mass only the nuclei of the sheath 

 can be distinguished from the others. The interior of the 

 epidermis-tube is now filled with supporting fibres. 



From the interior surface of the nucleiferous mass are 

 seen in parts nuclei wandering through the commissural 

 part of the brain, the „Punktsubstanz" ; these afterwards 

 form a cellular layer on the dorsal side of the commissures, 

 which had been hitherto only covered by the mesodermatic 



