No. 3.] THE EMBRYOLOGY OF THE EARTHWORM. 433 
apical plate (Scheitelplatte) is primitively the central organ of 
this sense-organ, and most observers agree that it is likewise 
median and unpaired. Kleinenberg’s remarkable studies on 
Lopadorhynchus and certain Phyllodoctde demonstrate, however, 
that in these forms the nerve-cells connected with the apical 
sense-organ form only a comparatively small part of the cephalic 
ganglia, the greater part being formed from several completely 
separate paired foundations. Kleinenberg’s observations, more- 
over, afford considerable ground for regarding the apical plate 
itself as having been primitively a paired organ; for in Lopa- 
dorhynchus it lies from the first on the right side, and in the 
corresponding position on the left side is a group of cells that 
soon disappears, and is suspected by Kleinenberg to represent a 
degenerate left apical plate. No other observer has studied the 
origin of the cephalic ganglia in the larval types with any 
approach to the thoroughness and care that characterizes 
Kleinenberg’s work, and until a more adequate study of the 
apical plate has been made in other larval types, its morpho- 
logical significance must remain in doubt. On the whole, 
therefore, it appears that no trustworthy basis for any funda- 
mental morphological distinction between head and trunk can 
at present be found in the mode of development of the cephalic 
ganglia. The adult cephalic ganglia in higher annelids certainly 
are paired, and it would seem that their mode of origin (whether 
from a paired or unpaired foundation) depends simply on the 
time at which the bridge of neural tissue between the two halves 
is formed, and this time is shown by the facts to be variable. 
As regards the second point, Kleinenberg lays great stress 
on the supposed independent origin of the cephalic and trunk 
ganglia, which he interprets (in accordance with the theory of 
development by substitution), as follows: The ring-nerve of the 
prototroch represents the ancestral central organ, and is the 
homologue of the ring-nerve of a medusa. In connection with 
it, in the course of the ancestral development, arose separately 
the cephalic ganglia in front (umbrellar region) and the ventral 
trunk-chain behind (sub-umbrellar region), the ring-nerve at first 
forming the connecting link between them, but subsequent!y 
disappearing in the adult and appearing only in the larva 
(Trochosphere). In the foetal types the ring-nerve has dis- 
appeared even from the larval stages, and the cephalic ganglia 
