No: 3.) THE ZMBRYOLOGY OF THE EARTHWORM. 415 
IX. Nervous SYSTEM. 
Apart from the discovery of the neuroblasts my results on the 
origin of the nervous system differ materially from those of 
Kleinenberg, and have, as I believe, an important bearing on 
the question as to the relation between the head and the trunk. 
Two principal questions are involved, namely: (1) do the cere- 
bral ganglia arise independently of the ventral chain, and (2) do 
they arise from a median unpaired apical plate (Scheitelplatte) ? 
As far as Lumbricus is concerned both these questions must be 
answered in the negative. My preparations show clearly, I 
think, that the entire nervous system has a double (bilateral) 
origin and is formed by a process of concrescence; and further- 
more, that the foundation of the cerebral ganglion of each side 
is simply the thickened anterior extremity of the corresponding 
neural row. 
In its first recognizable condition the nervous system is rep- 
resented (Fig. 47) by a short neural row lying at the surface of 
the germ-band on each side the body. Each row terminates 
behind in a neuroblast; anteriorly it passes up at the side of the 
mouth and is apparently lost in the general ectoblast. Its struc- 
ture in front can only be made out by the study of oblique lon- 
gitudinal sections passing in such a plane as to cut the neural row 
lengthwise (z.¢., a longitudinal plane inclined about 45 degrees 
to the sagittal plane). After many unsuccessful attempts I suc- 
ceeded in securing such a series, consisting of 27 consecutive 
sections through an embryo slightly older than that shown in 
Fig. 47. This series shows the relations so clearly and is of 
such importance that I figure six of the sections (Plate XX., 
Figs. 73-78). 
Figure 73 is the uppermost, z.¢., the one nearest the surface. 
It passes lengthwise through the posterior part of the germ- 
bands, cutting the primary mesoblast at J/, the right neuroblast at 
Vb, and showing the connections of these teloblasts with the cor- 
responding rows. The succeeding sections pass at successive 
lower levels, and, owing to the curvature of the germ-bands, show 
the forward continuation of the bands. Figure 74 is the section 
adjoining Fig. 73; it shows the neural row ze passing forwards 
and upwards at the surface of the body. Figure 75, two sections 
further on, is of great importance. It shows that the neural 
