History of a trausient nervous apparatus in certain Ichthyopsida. 345 



In the 4 th row a lengthy nerve occurs on the left side of the second 

 section, and in the 19*^ section there is a plexiform nerve on the 

 right. These are the only nerves in twenty-nine sections. 



In the twenty-five sections of the fifth row only one fibril, on the 

 right side of the ll^h and 12 th sections, was detected. 



Finally, in the twenty - three sections of the last row there is 

 practically a cessation of the nerves, only two short fibrils, both to 

 the right, being noticed. The embryo at this point is leaving the 

 yolk-sac, but is not yet quite off it. 



In this slide there are in various sections plenty of short fibrils 

 reaching to the skin, as in Fig. 76. 



Transient cells, or rather cells developing into such, were also 

 met with in mitotic division in this series. 



The development of the system is perhaps not completed. 



In embryo No. 552 (size about 10 mm) the number of somites 

 could not be counted — there were probably about 100 present. The 

 auditory organ is an open vesicle, and cells are being proliferated 

 from its epithelium into the applied ganglionic mass. 



The spiracle is still closed, the first and second branchials are 

 open, the third is a pouch reaching to the epiblast, and the fourth a 

 mere pouch not yet in contact with the skin. 



The oesophagus behind the gill pouches is closed. The optic 

 vesicles are constricted, and the invagination is just commencing. The 

 epiblast is slightly thickened on the site of the future lens. The 

 segmental duct does not yet reach the cloaca. 



There are indications of the transient system a little in front of 

 the pronephric region. These are met with some 25 — 28 sections 

 before the first pronephric funnel , but anterior to this latter there 

 appear to be vestiges of pronephric tubes. 



The first well-marked transient nerve lies some 28 sections anterior 

 to the first pronephric funnel, it is in the form of a fine fibril ex- 

 tending along the outer side of the myotome almost as far down as 

 the level of the segmental duct. Eleven sections further back the 

 top of the cord is seen to possess a cap of transient ganglion-cells. 



These are the sole very obvious elements of the transient system 

 anterior to the pronephros. In the following 21 sections in the 

 beginning of the pronephric region there are two pairs of transient 

 nerves, followed by a larger one, on the left side only, with ganglion- 

 cells near its basal portion and others in its course under the epiblast. 



In the next row of 28 sections there are four transient nerves, 



