History of a transient nervous apparatus in certain Ichtbyopsida. 347 



99 somites posterior to the last gill-pouch. In its other general 

 characters the present embryo resembles No. 562. 



. In the sections there are indications of the transient system for 

 some fifty sections in front of the pronephros, i. e. there are some 

 ganglion-cells scattered here and there on the cord, and at times 

 nerve-processes are encountered. These are not numerous. 



With the commencement of the pronephros elongated transient 

 nerves make their appearance; thus, 17 sections from the anterior 

 border of the first pronephric funnel there is a pair of such. In the 

 following 20 sections there are indications of three fibres, all to 

 the right. 



In the ensuing row of 19 sections there appear an elongated nerve 

 to the right side, 4 sections further on a pair of spun fibrils, after 

 yet other 3 sections an exquisite nerve (Fig. 88, Plate 26) passing 

 from a basal group of ganglion - cells along the outer side of the 

 myotome; after this a pair of fine fibres, and, finally, a single nerve 

 fibre on the left side. 



In the first row of the next slide the first four sections contain 

 a pair of elongated nerves proceeding from ganglion - cells on each 

 side of and outside (lateral of) the cord. 



Five sections further back there is a second pair of such nerves, 

 and then, in the remaining 17 sections, there are four nerves, two to 

 the right alternating with two to the left. 



One of these nerves is depicted in Fig. 87, Plate 26. Its com- 

 position reminds one of Fig. 24, but here the fibre is only in course of 

 development. There are 27 sections in the row. 



In the second row there is a single nerve (to the right) with 

 ganglion- cells at its base. A little further on the crossing of fibres 

 at the top of the cord, as in Fig. 76, is very obvious. One of the 

 spinning ganglion-cells of Fig. 78 a — g, is followed by two "ganglionated" 

 nerves, one to the right alternating with one to the left, which is 

 7 sections caudalwards; after other 6 sections there comes a nerve 

 with ganglion-cells in the myotome and others under the epiblast. 



There are 28 sections in this row. 



The following row of 23 sections only contains two nerves, both 

 to the right, and there is an interval of 13 sections between them. 



Near the top of the next row of 24 sections lie one of the spin- 

 ning ganglion - cells of Fig. 78 a— g, then in 3 sections a pair of 

 transient nerves, and some 10 sections beyond this a simple (left) 

 nerve. 



