432 A, A. W. HUBRECHT. 
and (2) nerve-cells has been followed by me in detail (1. c. (80), 
pl. iv, figs. 58—60, 64—71; pl. v, figs. 72—83, 87—89) in 
very numerous series of sections. They appear simultaneously ; 
and whereas a muscle-cell may soon be distinguished by the 
section of the fibril developing out of it, the nerve-cells at a 
very early stage give rise to the so characteristic fibro-nervous 
core which (as in the adult animal) the cellular constituents 
are found to surround, both in the lateral longitudinal stems 
and in the brain-lobes. I cannot with certainty say whether 
this fibro-nervous core, composed of extremely attenuated 
separate nerve-fibres, arises by the outgrowth of fibres out of 
pre-existent nerve-cells, or by the transformation of primarily 
cellular longitudinal strands into fibres. 
The very early period at which these fibres are distinctly 
visible, surrounded by embryonic cells, would make me incline 
towards the first view. I may, however, in still earlier stages, 
have overlooked rows of embryonic nerve-cells in the act of 
transition to nerve-fibres, because it is very difficult, at so early 
a period, to decide whether the embryroic cells are going to 
develope into nerve- or muscle-cells, and because this can in 
most cases only be answered with certainty after distinct 
fibres have made their appearance. 
As well in the prostomial cell mass, as in the mesoblastic 
layer of the posterior region of the body, such fibrous cores are 
thus demonstrable at a very early period. In the prostomial 
mass they are from the very first arranged as they are in the 
adult, i. e. in two ventral brain masses, from which the lateral 
cords spring, and two dorsal ones forming the superior brain- 
lobes. Anteriorly the lobes coalesce right and left, and both 
halves are again united by an annular commissure, surrounding 
the proboscis and its sheath, and also observable in a very early 
phase (1. c. (30), pi. v, fig. 81). It is absolutely impossible, 
in all the numerous series of sections which I possess of these 
early stages, to find one single instance that might be adduced 
in favour of an epiblastic origin of the nervous system. It 
must be borne in mind that it would be the secondary epiblast in 
which the process of the origin of the nervous system would 
