346 JOHN BEARD, 



of these three are on the left side and one only on- the right. Of 

 these only the second and third appear to form a pair, of which the 

 one on the right is much bigger than its fellow. 



In the next 26 sections there are again four transient nerves. 



The first two of these lie on the left side and one of them is a 

 fine one reaching down the length of the myotome. The last two, 

 situated near the end of the row, appear to form a pair. 



In the 23 sections of the succeeding row there are only two 

 transient nerves, both on the left side, and the second is much the 

 larger ; it has ganglion-cells near its base, as well as one in its course 

 under the epiblast. 



Four transient nerves are seen in the 25 sections of the ensuing 

 row ; of these the two anterior may perhaps form a pair, the remaining 

 ones lie both on the left side. 



The next fact to be noticed is a decussation of fibres between 

 centrally lying ganglion - cells, as in Fig. 76. Moreover, it should be 

 mentioned that in this row there are two cases of mitotic figures of 

 cells of the transient system. There is in this series of 21 sections 

 only one transient nerve, and that a fine one, on the left side. 



Altogether there are five transient nerves in 20 sections of the 

 next row. The first of these lies to the right side, as does also the 

 second. The third and fourth form a pair, of which the third is much 

 larger than the fourth, although both have a prolonged course. The 

 fifth nerve lies to the left. 



After 20 sections more only one additional nerve has been en- 

 countered, it appears to be the last of the lot and lies to the left 

 side. In the succeeding sections there are no nerves. 



A summary of the nerves of this series is as follows : — 



There are 21 nerves, paired or unpaired, in all, as well as many 

 small fibres, there are 14 unpaired nerves on the left side and only 

 4 on the right. These are all that were seen in 211 sections of about 

 Vi3 8 mm, i. e. the nerves do not extend over quite 2 mm of the 

 embryo ^). 



Embryo No. 564 (size about 10,5 mm) showed externally traces 

 of five gill pouches or clefts. In the sections it is seen that of these 

 the first and second branchials possess openings to the exterior, on 

 one side the third branchial has opened, but it is still a pouch on 

 the other. The fourth branchials are in the form of gut-pouches, and 

 the spiracle is still without opening on either side. There are about 



1) A very significant fact. 



