History of a transient nervous apparatus in certain Ichthyopsida. 351 



An analysis was made of twelve rows of sections of this series, 

 beginning with one in which the most anterior pronephric funnel was 

 cut through and ending some little way in front of the point where 

 the ganglion-cells ceased. 



In all there were about 214 sections of about ^/ij., mm con- 

 tained in these twelve rows, and in every section ganglion-cells were 

 encountered. Often there were five or six in each section, occasionally 

 they were reduced in number to two or three. 



Some idea can thus be gathered of the great abundance of the 

 cells in the region, to be afterwards defined, in which they occur in 

 the early embryos of. R. hatis. 



As compared with embryo No. 141 the poverty in sub-epiblastic 

 nerves is a noteworthy feature of the present embryo. And this leads 

 at once to the remark, which occurs to one after the examination of 

 several embryos of B. hatis ranging in size from 8 to 12 mm, and 

 also of such which, even when apparently of the same age, vary in 

 their characters, that the transient system develops in- 

 dependently of the development of the embryo. 



To repeat, this series (No. 180) exhibits no remarkable advance on 

 No. 141 except in the number and state of activity of the ganglion- 

 cells. Of these there appear to be many still in statu naseendi. The 

 transient nerves, though pretty numerously represented, offer nothing 

 like the pictures described from Nos. 141, 410 and 419. 



For the rest, the ganglion-cells are now acquiring capsule-cells 

 from other elements of the summit of the cord as shown in Fig. 55. 



Embryos Nos. 156 and 157 (11 — 12 mm) are almost of the same 

 age. In No. 156 the number of somites is about 100, and external 

 gill-buds are present on two arches, the last two pouches do not open 

 to the exterior, there are external gill-buds on three arches and about 

 104 somites posterior to the last open gill- cleft could be counted. 



The optic cup is invaginated and, while in No. 157 the lens is 

 becoming invaginated, in No. 156 it is already a vesicle. In both the 

 neurenteric canal is still open. No. 157 forms a series of transverse 

 sections. No. 156 a series of horizontal ones. From the latter a por- 

 tion of a section of the summit of the spinal cord is represented on 

 Plate 27 Fig. 99. The crowding of the transient ganglion-cells is 

 better revealed by some other figures of horizontal sections. Here 

 they are not very numerous. A projection of ganglion-cells (sp.c) in 

 the act of spinning will be noticed, as well as a small group of such, 

 not connected in the present section with those on the cord. 



