— 14 — 



only distinguishes itself from the last by the presence of 

 a slight depression on its surface. 



After the next "surface pit" has been reached, the 

 germ-layer loses its continuity and breaks up into a 

 number of groups of cells, each lying in front of its 

 corresponding "surface pit". The space around these 

 groups is filled with nerve-fibres, forming the commis- 

 sures; they seem to run in every direction. A change 

 has also taken place in the epidermis; its nuclei are no 

 longer so elongated, having become more rounded ; they 

 now measure '01 mm. by '005 mm. 



The first of the groups of brain-germ cells met with 

 measures '08 mm. in length by *02 mm. in breadth, and 

 lies *02 mm. from the surface of the embryo. It is 

 composed of nuclei lying about three deep and twelve 

 broad, measuring between *009 mm. and *01 mm. They 

 all lie with their long axes pointing towards the surface. 

 The nuclei of these groups are all connected with the 

 centre of the "surface pit" by radial fibres, which all 

 converge towards it. The fibres probably belong to the 

 neuroglia, and serve as a supporting tissue. 



The epidermis is discontinued just at this "surface 

 pit", as it was in the last. The "surface pit" also 

 resembles the last described in having a slight depression 

 on its surface. The cells of the epidermis also take an 

 active part in the building up of the groups, by forming 

 a funnel-like sheath around the radial fibres. The nuclei 

 of the cells forming this sheath measure about *018 mm. 

 in length by .005 mm. in breadth. Karyokinetic figures 

 are sometimes to be found in them. 



The next such collection of germ-cells is, perhaps, 

 even more typical. It lies just at that point where the 

 abdomen forms an angle to the cephalotorax (fig. 8). 

 Although more advanced than the last its dimensions are 

 not larger. 



The length and breadth are the same as those of the 

 last group, the length of the radial fibres being '02 mm., 

 as in the last case; the group, however, now lies 

 •037 mm. from the surface. 



