FrenzcVs Mesozoon SalincUa. 475 



direct stimulus to it. The p;rcat caducity of endoderm cells 

 (and of gland-eel U altogether) is a character of very general 

 occurrence. 



Were we able, says Frcnzel, "still to regard the latter [i. e. 

 the intestinal cells) at all events as Protozoon cells, this view 

 would be absolutely inadmissible for the former, the cells of 

 the mesoderm and ectoderm . . . ." 1 do not at all see why. 

 The intestinal cells with intra-ccllular digestion correspond to 

 holo])hytie Protozoa ; the rest of the cells of the body corre- 

 s])ond partly to sa])roj)hytic Protozoa, because, thanks to the 

 labours of other cells, they need only to feed, but not to digest 

 their food ; in part, however, the body-cells (especially those 

 of the mesenchyma) are likewise holophytie Protozoa, and 

 remain so even when the intestinal cells have long lost the 

 faculty of intra-ccllular digestion. In more primitive cases 

 the intestinal cells themselves digest ; they subsequently lose 

 this faculty, and henceforth expend their energies in the pro- 

 duction of digestive juices; the latter, however, are not suffi- 

 cient for the digestion of the food-matter, and the wandering 

 amoeboid cells have to assist more or less with their power ot 

 intra-cellular digestion. The intestinal cells continue to be 

 Protozoa, with which other body-cells, likewise corresponding 

 to Protozoa, live together in a kind of symbiosis : their 

 sci'vices to one another are reciprocal, so that their functions 

 are consolidated into a physiological whole. Not only do the 

 intestinal cells feed the rest, but a large portion of the latter 

 also make provision for the intestinal cells : oxygen is in the 

 widest sense food, just as much as albumen, fat, and carbo- 

 hydrates. 



I believe that I have sufficiently demonstrated in the fore- 

 going that it is precisely the physiology of digestion which 

 causes least difficulties in deriving the Metazoa from the Pro- 

 tozoa ; but also the other "gulf" between Protozoa and 

 Metazoa, which Frenzel likewise emphasizes, and which is 

 occasioned by the multilaminar character of the Metazoa, 

 appears to us less great when we take into consideration the 

 following facts. 



As a single-layered multicellular animal we are now also 

 acquainted with Salinella, besides Volvox. The next stage, 

 with the representatives of which we have closer acquaintance 

 as adult animals, already consists — to leave Trickoj^lax ad- 

 luerens out of consideration — of three layers, since in them a 

 mesoderm, or, better, mesenchyma, is already present between 

 ectoderm and endoderm ; for of animals which in the adult 

 vStage also would correspond to the typical Gastrula, and con- 

 sist merely of ectoderm and endoderm, we have no knowledge. 



