154 THE GERM-PLASM 



CHAPTER IV 



MULTIPLICATION BY GEMMATION 

 I. The Process of Gemmation in Animals 



If, with von Wagner, we look upon gemmation as 'a process in 

 which entire individuals are formed anew/ and which depends 'ex- 

 clusively on a special (differential) growth differing from the nor- 

 mal one,' we must include under this term the processes of asexual 

 multiplication which occur in most of the Caloiterata, the Polyzoa^ 

 and the Tunicata. 



A . — Ccelenterata 



Hitherto it has been considered that we were fully acquainted 

 with the process of gemmation in the Cceletitarata. especially in 

 the case of the Hydrozoa. It had been observed that the two 

 layers of cells which form the body-wall of these animals are 

 present even in very young buds of Medusas and Hydroid- 

 polypes. These layers surround the digestive-cavity just as they 

 do in the parent animal, and since the body-wall as well as the 

 cavity it encloses are in direct connection with those of the 

 parent, nothing was more natural than to suppose that the bud 

 arises as an evagi7iation of the body-wall of the parent, both layers 

 of the latter taking part in its for7)iation from the first. A doubt 

 as to the correctness of this statement was less likely to arise 

 owing to the fact that even in the youngest buds of a Hydroid- 

 polype, before they become hollow, the ectoderm and endoderm 

 were seen to consist of a number of cells engaged in active 

 multiplication. I myself made such a statement in connection 

 with my investigations on the formation of the sexual cells in 

 Hydroids,* and no doubt has yet been raised as to its correct- 

 ness, or rather as to its interpretation. 



The assumption that both germinal layers of the parent take 

 part in the formation of the bud is nevertheless an incorrect one ; 

 for the bud arises from the ectoderm only., and the young cells 



* ' Die Enstehung der Sexualzellen bei den Hydromedusen ' (with 25 

 plates), Jena, 1883. 



