146 BULLETIN OF THE 



the glands of the foot, then through the mass of indifferent mesenchyme 

 which lies under the dorsal ectoderm at >|c. Just in front (IV. $ ) 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 coecum 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 papillae. 

 As a matter of fact we do find it, as is shown in Figure 16 at the base of 

 cerata II. and III. (^). 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 ^Eolis points 

 emphatically to one conclusion, namely, the embryonic or growth tissue 

 of vEolis is in its origin identical with that producing sexual cells. Like 

 the latter, 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 eutodermal diverticulum to the entoderm of the buds. 



