146 BULLETIN OF THE 
the glands of the foot, then through the mass of indifferent mesenchyme 
which lies under the dorsal ectoderm at *. Just in front (IV. g ) sexual 
cells are being cut off from the mesenchyme as a paired mass whose two 
lobes are united in the median plane. 
I have above assumed, somewhat gratuitously, that the mesenchyme 
takes the initiative in ceras production. The evidence for this lies in 
two facts. (1.) The first indication of the formation of the new ceras 
is seen in the thickening of the mesenchyme at the base of the next 
older ceras (Fig. 5). It is not until after a solid mass of mesenchyma- 
tous cells is produced that the ectoderm begins to evaginate, almost as 
though pressed outwards (Fig. 6). The alimentary diverticulum is pro- 
duced still later (Fig. 7). (2.) That the eccecum does not take the ini- 
tiative is indicated by the fact that I have found young cerata composed 
only of ectoderm and a thickened mesenchymatous core, the entoderm 
not having yet penetrated into it. 
The capacity possessed by Nudibranchs of regenerating the cerata is 
well known. I have not experimented with them, and have no sections 
of stages in the process. The known phenomena of regeneration in other 
cases makes it probable that the capacity for regeneration depends upon 
the existence of embryonic tissue. We should therefore expect to find 
thickened, embryonic mesenchyme lying at the base of the dorsal papille. 
As a matter of fact we do find it, as is shown in Figure 16 at the base of 
cerata II. and Il. (). The mesenchyme at the base and in front of 
ceras I. was torn away in sectioning; in adjacent sections the basal 
mesenchyme appears thickened here also. 
The foregoing study of the development of the cerata of Molis points 
emphatically to one conclusion, namely, the embryonic or growth tissue 
of Aolis is in its origin identical with that producing sexual cells. Like 
the hatter, it is germ tissue; it differs from the sexual cells chiefly in this, 
that it gives rise to growths constituting part of the body of the present 
individual, — growths which are as mortal as any other part of the pres- 
ent individual ; whereas the sexual cells play no part in the production of 
the present individual, but eventually give rise to a new individual and 
its germ tissue. It differs, secondly, from the sexual cells in this, that it 
gives rise to one kind of organ only, —the mesenchyme to the mesenchyme 
of the buds, the entodermal diverticulum to the entoderm of the buds. 
