1^6 THE GERM-PLASM 



form a layer of young actively dividing cells, such as I had for- 

 merly observed in very young buds : this layer forces the older 

 endoderm cells away from the supporting membrane, in conse- 

 quence of which they loosen their connection with the rest of 

 the endoderm, undergo disintegration, and gradually become 

 absorbed. The cells, however, which have migrated from the 

 ectoderm then give rise to the endoderm of the bud. 



Now that these facts have been proved by Lang's investiga- 

 tions,* it is easier to give a theoretical explanation of the process 



■H/cP 



■EKt 



Fig. 6. — Diagrammatic section through the rudiment 

 of a bud oi Etidendrium. (Modified from a figure 

 by Albert Lang.) Ps, the horny perisarc; Ps , 

 portion of the perisarc which has become very 

 thin owing to the proliferation of the underlying 

 ectoderm {Ekt') ; Ent' , the region of the endoderm, 

 in which a number of proliferating ectoderm cells 

 have broken through the supporting lamella {st), 

 migrated into the endoderm, and caused the latter 

 to project into the gastric cavity. 



of gemmation in Hydroids. We must, however, still assume 

 that certain cells and series of cells in the ectoderm are provided 

 with an accessory idioplasm, which contains all the determinants 

 of the species, and which is therefore a kind of germ-plasm, 

 though perhaps not quite identical with the germ-plasm proper : 



* Albert Lang, ' CJeb. die Knospung bei Hydra u. einigen Hydro- 

 polypen,' Zeitschr. f, wiss. Zoo!., Bd. 54, 1892, p. 365. 



