342 JOHN BEARD, 



best and highest lenses, for with ordinary powers there appeared to 

 be only one cell here. 



A sketch of this by my pupil, Mr. J. A. Mureay, will be found 

 on PI. 26, Fig. 24 a. 



The cell gl. c^ gives off a fibre (s. n) which passes down in the 

 same way as that of the other side. The remarkable nature and 

 the close similarity of these two nerves are very striking. 



Another combination from the same series is given in Fig. 25. 

 Here on the left side the nerve (s. w), when followed through three 

 sections, was seen to have exactly the extension given to it in the 

 drawing. How much further it proceeded was an insolvable problem. 



The pieces of the nerves were, as just stated, completely present 

 in one or other of the three sections. Thus, the 6 th section of the 

 row furnished the "root" piece to nearly the tip of the myotome, as 

 also a small bit opposite the segmental duct, the Tt'i section revealed 

 a piece near the tip of the myotome, the continuation ot that in the 

 6th ^ and the 8^'» section contained a long intact stretch of fibre reach- 

 ing from just above the segmental duct to nearly the top of the 

 myotome. 



It is very curious, — might one not add significant? — to note 

 that these nerves are most apparent in the region where the em- 

 bryonic layers pass on to the yolk-sac, and that where the body is 

 shut off from the yolk, as in the next slide of the series, they cease, 

 or, at any rate, have not so great an extension ^). 



There are many nerves like those of Figs. 23 and 25 in this 

 series, but others were noted which had the same composition as 

 those in Fig. 24. 



In embryo No. 562 (size 10 mm) there were four pouches or 

 clefts externally visible. In the sections it is seen that of the gill- 

 clefts the first and the second branchials are open to the exterior, 

 the spiracle is still closed. The primary optic vesicles are con- 

 stricted, but as yet not invaginated. The auditory depression is 

 widely open. The segmental duct extends some way back, but as yet 

 it does not reach the cloaca. 



In the region of the pronephros the cells of the future transient- 



1) This point is more fully discussed in a foot-note in a subsequent 

 section. The above was written before a suspicion of the cessation of 

 the nerves, where the embryonic body quitted the yolk-sac, became 

 abundantly verified by observation in other embryos. 



