284 PASSAGE FRO.lf THE PROTOZOA TO THE UKTAZOA. 



inaugurated. It may be noted that an iniracelhilar protozoon type of 

 digestion persists in the Porifera, and appears also to occur in many 

 Ccelenterata, Turbellaria, &c., though in most of these cases both kinds of 

 digestion probably go on simultaneously 1 . 



Another hypothetical mode of passage, which fits in with delamination, 

 has been put forward by Lankester, and is illustrated by tig. 205. He 

 supposes that at the blastosphere stage the fluid in the centre of the colony 

 acquired special digestive properties ; the inner ends of the cells having at 

 this stage somewhat different properties from the outer, and the food being 

 still incepted by the surface of the cells (tig. 205, 3). In a later stage of the 

 process the inner portions of the cells became separated off as the hypo- 

 blast; while the food, though still ingested in the form of solid particles by 

 the superficial cells, was carried through the protoplasm into the central 

 digestive cavity. Later (tig. 205, 4), the point where the food entered 

 became localised, and eventually a mouth became formed at this point. 



The main objection which can be raised against Lankester's view is 

 that it presupposes a type of delamination which does not occur in nature 

 except in Geryonia. 



Metschnikoff has propounded a third view with reference to delamina- 

 tion. He starts as before with a ciliated blastosphere. He next supposes 

 the cells from the walls of this to become budded off into the central 



Fuj.t 



EC 



FIG. 205. DIAGRAM SHEWING THE FORMATION OF A GASTRULA BY DELAMINATION. 



(From Lankester.) 



Fig. 1, ovum; fig. 2, stage in segmentation; fig. 3, commencement of delamination 

 after the appearance of a central cavity; fig. 4, delamination completed, mouth form- 

 ing at M. In figs. 1, 2, and 3, EC. is ectoplasm, and En. is eudoplasm. In fig. 4, 

 EC. is epiblast, and En. hypoblast. E. and F. food particles. 



1 J. Parker, "On the Histology of Hydra fusca," 1 Quart. Jnurn. Micr. Science, 

 vol. xx. 1880; and El. Metschnikoff, "Ueb. die intracellulare Verdauung bei Ccelente- 

 raten," Zoologischer Anzeifjer, No. 50, vol. in. 1880 and Laukester, " On the intracellu- 

 lar digestion and endoderm of Limnocodium," Quart. Jmirn. Mirr. Science, vol. xxi. 

 1881. 



