INTRACELLULAE DIGESTION OF INVERTEBRATES. 109 



lower Coelenterates we can even yet hardly speak of a raeso- 

 blast. Though many Acraspeda have amoeboid cells in their 

 gelatinous tissue, others have no trace of such elements ; and 

 their presence in Craspedota is altogether exceptional. It is 

 worthy of note that in Medusse these cells, when present, 

 appear only late in life.. If we consider the supporting cells 

 of the tentacles in Medusse and hydroids as mesoderm, we are 

 still unable to draw a line between them and the endoderm. 



In higher forms, the distinction between the two layers is 

 much clearer ; the endoderm has assumed the function of 

 utilising the food brought to it from without, and an intra- 

 cellular absorption is gradually replaced by a process of 

 enzymogenesis (a change which will be more fully discussed in 

 the third section of this work). The mesoderm does not, 

 however, lose its primitive powers, but employs them against 

 useless and harmful bodies, so that it retains the intra- 

 cellular digestion, as well as many other of the characters of 

 the Protozoa — not only the power of throwing out Pseudo- 

 podia, but also that of forming Plasmodia, This last pro- 

 perty has persisted less in the ectoderm than in the other 

 layers, being only seen in the epidermis of Sponges, Hydroids, 

 and perhaps a few other Coelenterates ; it is well preserved 

 in the endoderm of animals with an intracellular digestion, 

 such as Coelenterates and Turbellarians. Mesodermal Plas- 

 modia are, however, found even in the higher animals, not 

 excepting Man himself. The cells of the mesoderm have 

 best preserved their primitive independence one of another, 

 so that one can truly say that the protopsychic condition 

 here persists. 



Many embryologists, in their endeavours to elucidate the 

 history of the middle layer, have attempted to determine its 

 special function. Some (Hatschek) have believed it to be 

 specially connected with reproduction, while others (Rabl) con- 

 sider that it arose in connection with a locomotive apparatus. 

 Considering the facts described in this paper, and, above all, if 

 we remember that in many animals with a mesoderm both 

 generative organs and musculature are derived, not from this 



