344 JOHN BEARD, 



indications of two other pouches. Only one gill-cleft is- actually open, 

 i. e. : — the first branchial. There are about 106 somites. The 

 auditory invagination is still widely open. The optic vesicles are 

 somewhat constricted, and there is a very slight epiblastic thickening 

 on the site of the lens. 



The first traces of the transient system are encountered some 

 twenty sections (of about ^/goo ^^ thickness) in front of the pro- 

 nephros. Here they take the form of occasional ganglion-cells spinning 

 out short processes, but, as the pronephros is being reached, ganglionic 

 groups, forming extended nerves, are seen on the right side. A few 

 sections further ou a pair of such nerves is very prominent. In this 

 row there are in all twenty-nine sections. 



At the very commencement, i. e. in the first three sections of 

 the following row, a pair of transient nerves, as fine as any of those 

 depicted in the plates, comes to view, and in the remaining twenty 

 sections of the row there are three other nerves on the right and a 

 curious chain of ganglion-cells. The pronephros now terminates. The 

 following row of twenty-two sections reveals a large transient nerve 

 to the right side, and, 10 sections further on, a pair of such nerves. 

 Both proceed from ganglion-cells projecting from the cord. 



The fifth row of this slide reveals a spinning ganglion- cell like 

 those of Fig. 78. In some of its sections the crossing or decussation 

 of fibres on the top of the cord is quite obvious, as in Fig. 76. Then 

 in the 8^'' section there is a long fine fibril extending down the outer 

 side of the myotome on the right. And in the 17 t^ and 18 ^^ sections 

 the beautiful pair of nerves of Fig. 75 is contained. In all there 

 are twenty-three sections in the row. 



In the twenty-four sections of the first row of the next slide are 

 the following : — A large transient nerve on the left with ganglion- 

 cells at its basis, a spinning ganglion-cell, and, finally, in the next 

 section a transient nerve, with ganglion-cells at its root, which possibly 

 completes the pair. 



In the second row there are twenty -two sections and three 

 pairs of transient nerves. Of these the first has a coating of ganglion- 

 cells at the root-end, whilst the other two are free from such cells 

 and proceed from ganglion- cells on the top of the cord. 



In the third row there is already an apparent diminution in the 

 number of nerves ; for in 33 sections there are only one pair of nerves 

 with ganglion-cells at the root, a similar single one on the right, and, 

 18 sections from this, a fibril on the left. 



