MULTIPLICATION BY GEMMATION 155 



which form the endoderm in these buds are not derived from the 

 endoderm of the parent, but have migrated from the ectoderm. 



Purely theoretical considerations first led me to suppose that 

 this must be so. The origin of the process of gemmation in the 

 idioplasm can only be brought into agreement with the theory 

 of the continuity of the germ-plasm, if the cells of the parent- 

 organism from which buds arise collectively contain all the 

 determinants of the species as accessory idioplasm. If this were 

 not the case, an entire animal, capable of reproduction, could 

 never arise from the bud. If, now, a certain cell of the ectoderm 

 contained all the determinants for the outer, and one of the endo- 

 dermal cells all those for the inner layer, a bud could only be 

 formed when these two cells happened to lie exactly opposite to 

 one another in the body-wall. As, however, the endoderm cells 

 form a definite and continuous epithelial layer, and have a fixed 

 relative position, and, moreover, the position of the ectoderm 

 cells, although not quite so definite, is still on the whole a fixed 

 one, I found it difficult to imagine how budding could take place 

 at perfectly definite parts of the polype and of the stock in such 

 a regular manner as actually occurs in many cases. The assump- 

 tion that all the cells of the ectoderm and endoderm are equally 

 provided with the necessary accessory idioplasm is excluded by 

 the fact that budding occurs in such a regular manner. I am 

 therefore led to suppose that the distribution of the ' blastogenic * 

 germ-plasm ' might possibly be confined to one germinal layer 

 only ; and since it is known that in Hydroids the germ-cells are 

 always developed from the ectoderm, it is natural to conclude that 

 the blastogenic idioplasm is contained in the cells of this layer. 



This conclusion has now been confirmed by investigations 

 carried out by Mr. Albert Lang, in the Zoological Institute at 

 Freiburg. In various Hydroid-polypes {Eudendriu}n, Plunm- 

 laria, and Hydra) the bud arises in the following way. The 

 cells in a certain small circumscribed region of the ectoderm 

 first begin to multiply, the ' supporting lamella,' which separates 

 the two layers of the body-wall, gradually becoming thinner and 

 softer at the same time, and then a few of the newly-formed cells 

 penetrate into the endoderm through this membrane. Here they 



* The term ' blastogenic idioplasm ' is here used in the special sense of 

 ' Knospungs-Idioplasma," and not in the more general sense in which 

 it is usually used by the author (cf., e.g., the chapter on ' The Supposed 

 Transmission of Acquired Characters'). — W. X. P. 



