History of a transient nervous apparatus in certain Ichthyopsida. 355 



Another embryo of 17 mm (No. 507) is very similar in the average 

 of its characters, including those of the transient system. 



I pass over two other embryos, of 18 and 19 mm respectively, 

 because no sections from them have been figured , and , more parti- 

 cularly, because they present no very noteworthy features in the 

 transient system. There is a disadvantage as well as a god-send in 

 securing an extensive embryological material for an investigation. A. 

 wealth of this kind is not always an unmixed blessing, for it is dif- 

 ficult to decide that the work has gone far enough for final publi- 

 cation, even after years of labour, when the supply of interesting 

 material, which may still, it is hoped, throw fresh light on the subject, 

 is unexhausted. In such cases the observer may be exhausted long 

 before his prepared material. The reader must not suppose that the 

 embryos described are the sole ones that have been studied — had it 

 been needful to have delineated all those examined, the difficulty ex- 

 perienced in getting done with the research would have become well- 

 nigh an impossibility. 



Embryos of 20 mm demonstrate practically the same features in 

 the transient system as some of those of 21 mm and upwards to be 

 described presently. From one such embryo (No. 48) Fig. 35, pi. 23 is 

 taken. What is depicted is of some little importance in view of subsequent 

 theoretical considerations. As in Fig. 31, previously described, there 

 is a bipolar ganglion-cell {w. gl. c) of the transient system lying in the 

 "mesoderm" just upon the cells of a spinal ganglion. Each end of 

 this ganglion-cell is spun out into a long nerve-process, the one of 

 these processes being in touch with ganglion-cells in the cord, the 

 other arching over the myotome. In this instance, as the finest lenses 

 reveal, there is no continuity of the two axis- cylinders through the 

 cell, such as is demonstrated by Fig. 31. 



4. Embryos of 21 — 25 mm. 

 The first embryo (No. 189) to be mentioned calls for no detailed 

 consideration, as only one section from it is figured (plate 24, Fig. 56). 

 This embryo measured nearly 21 mm. Small external gills are present 

 on all five of the posterior arches ^). The neurenteric canal has not 

 yet vanished, but is near the point of doing so. In the whole embryo 



1) Raja embryos never at any period possess external gills on the 

 mandibular arch in connection with the spiracle. Johannes Müller 

 noted this fact in 1840 (see "Ueber den glatten Hai etc.", in : Abh. 

 Akad. Berlin, 1840, p. 250). 



Zool. Jahrb. IX. Abth. f. Morph. 24 



