History of a transient nervous apparatus in certain Ichthyopsida. 363 



ponents, each process would find its starting-point in one or other 

 of the ganglion -cells. Figs. 82 and 100 are from sublimate preparations, 

 whilst those of embryo No. 214 (Figs. 83 and 92) originate from a 

 period when Rabl's picric-platinum-chloride mixture was the only 

 reagent employed in preservation. There is little to describe in these 

 two figures. Fig. 92, pi. 27 shows a number of the transient ganglion- cells 

 from the summit of the cord in embryo No. 214 (33 mm) as seen in 

 horizontal sections. No processes are shown, a deficiency made good 

 by Fig. 82, pi. 26, but from the figure some idea may be obtained of the 

 nuclear contents, and also of the topography of the cells. Fig. 83, 

 plate 26, is once more an example of a "mesodermal" bi-poiar ganglion- 

 cell of the transient system. 



Owing to certain disturbing factors, i. e., the great abundance of 

 sub-epiblastic connective tissue, and the numerous nerve twigs and 

 developing branches of spinal and lateralis nerves, it is difficult to 

 be sure that these simple nerves have really disappeared entirely. 

 From this to later stages the investigation of the transient nerves 

 becomes increasingly laborious. 



6. Embryos of 42 — 45 mm. 



Under this heading three embryos will be dealt with. The meas- 

 urements are: — No. 227, 42 mm. No. 229, 43 mm, and No. 237, 

 45 mm. 



No. 229 measured from head to pelvis, both included, 15,5 mm. 

 General characters: The thymus-elements ') are still attached to the 

 epithelium of the gill-clefts as pear-shaped structures. 



The external gills vary in size from 5 mm in No. 227 to 7 mm 

 in No. 237. In all the oesophagus is still closed. The embryos are 

 practically identical in the characters exhibited by the transient 

 system. 



Owing to growth backwards of the gill-region, the ganglion-cells 

 appear for the first time opposite its posterior part, and extend well 

 into the region opposite the beginning of the genital organs. Capsule 

 cells are present in connection not only with the centrally lying 

 ganglion-cells, but also with such ganglion-cells as lie singly or in 



1) The thymus thickenings first appear in embryos of 20 — 21 mm, 

 the proliferation of leucocytes begins in embryos of 26 — 29 mm, and 

 the elements become freed from the gill-epithelium in embryos of 70 mm 

 or thereabouts. 



