XVII PEOTOCHOEDATA 645 



consists of cells in a plastic condition, and all the organs of the bud 

 must arise from it, since endoderm in the bud is conspicuous by its 

 absence, i.e. there is no inner layer. 



_ Another way of looking at the matter, however, occurs to one on 

 thinking over the lessons learnt from the development of the egg of 

 Cynthia. We have learnt that the cytoplasm of the egg in this 

 form contains specific organ - forming substances; and that the 

 germinal layers and the rudiments of the organs in the developing 

 embryo are distinguished from one another by the possession of 

 different organ-forming substances. We can see this in Cynthia ; the 

 presumption is that such substances distinguish the germinal layers 

 from one another in the case of embryos right throughout the animal 

 kingdom, though the chemical differences between them do not usually 

 reveal themselves to our eyes by different colours. The formation of 

 these substances we have seen to be the work of the nucleus of the 

 developing oocyte, while their final arrangement is due to the sperma- 

 tozoon, or rather, we might say, to the zygote-nucleus, for the sperma- 

 tozoon and egg-nucleus come together by a mutual attraction. But, 

 once the definite location of these substances is fixed, it appears as if 

 the nuclei of the segmenting egg were powerless to affect them. 

 Pressure experiments on the eggs of Echinoderms, and even on highly 

 speciahzed eggs hke that of Nereis, show that these nuclei can be juggled 

 about like a handful of marbles, and made to change places with each 

 other without affecting the disposition of the organs of the embryo. 

 Thus the specific quality of organs depends on the cytoplasm of the 

 egg, and not on its nuclei. 



But it seems certain that this passive condition of the nuclei is 

 only a transitory phase. Leaving aside the obvious fact, that when 

 the generative cells are formed the nucleus must resume its active 

 role, there is evidence to show that the specific ferments which 

 characterize certain gland cells, such as the pancreatic cells, are 

 formed out of particles emitted by the nucleus. It is then reason- 

 able to assume that, in the formation of buds, the nuclei again become 

 active and manufacture organ-forming substances in the cytoplasm, 

 but it does not follow that the arrangement of these will be the same 

 as in the egg. It may be that the possibility of the formation of 

 these substances is bound up with a certain quaUty of the cytoplasm, 

 and the nuclei in the outer layer of the bud may attempt to do this 

 and may fail. We may reflect that in Ciona, if any of the cells of 

 the developing embryo be killed, a portion of the tissues of the larva 

 will be missing, and that the larva is unable to make good the defect ; 

 and that yet, if the larva be allowed to metamorphose into an Ascidian, 

 and the whole upper part of the young Ciona, including the ganghon, 

 be bitten off, the stump can regenerate the missing parts. That 

 being so we can realize the difference between an active and an 

 inactive condition of the nuclei. 



It has been hinted above that, since Ciona possesses two large 

 thin-walled epicardia, the view has been held that it and the other 



