340 JOHN BEAKD, 



A single drawing (Fig. 32) is given from No. 163. It is a com- 

 bination of what is contained in three consecutive sections. There 

 are two sub-epiblastic nerves (s.w), each the product of two ganglion- 

 cells near the centre, and the one to the right differs from its fellow 

 in that some distance along its course two ganglion-cells (gl. c), lying 

 closely beneath the epiblast, are applied to it. As in other cases the 

 nerves themselves have been drawn in to the exact extent of their 

 preservation in the sections. 



Embryos Nos. 410, 411 >) and 419 2) all belong to this period. 

 Nos. 410 and 411 measured 10 mm in the preserved state, No. 419 

 was 10,5 mm in length. In the latter the lens is only represented 

 by an epiblastic thickening, in the two others there is a slight in- 

 vagination. The last gill-pouch does not open to the exterior, the 

 segmental duct is not completely formed, and the neurenteric canal 

 is still open. No figures are given from No. 411, but it may be 

 mentioned that in it the transient ganglionic system is in much the 

 same condition as in No. 410, and that it also presents similar sub- 

 epiblastic nerves. 



Plates 22 and 23 contain four figures drawn from No. 410 (Figs. 

 22 — 25). Over the region of the cord to be defined at a later stage 

 the ganglion-cells of the transient system are met with in prac- 

 tically every section. 



As seen in the figures, from 6 to 8 ganglion-cells may be en- 

 countered in each section. Like the other figures of the series Fig. 

 23 is a combination from several sections. It depicts the course of 

 two sub-epiblastic nerves (s. n) from cells of the cord, over the myo- 

 tome-tips, along the outside of these structures, and, immediately 

 beneath the epiblast, for some considerable distance past the segmental 

 duct. There can be little doubt that here the nerves become "extra- 

 embryonic", and pass on to the yolk-sac. The two nerves under con- 

 sideration presented no nuclei at any part of their course. They are 

 simple naked uon-medullated fibres, or if they possess any "white 



1) In No. 411 there are about 104 somites, the first four clefts are 

 open, and the fifth and sixth are present as gut-pouches, in No. 419 

 there are about 90 somites, the spiracle is still closed, but the first 

 and second branchial clefts are open. 



2) These three embryos were all treated with Flemming's osmic 

 mixture. This reagent, apart from the brittleness it gives rise to, is 

 an excellent one for making preparations of these nerves. It appears, 

 however, to possess no special advantages over corrosive sublimate. 



