ANATOMY OF THE MADEEPORAEIAN COEAL FUNGI A. 313 



to denote the outer and inner limiting layers of the adult 

 body, whilst the name mesoderm should include all those tis- 

 sues which lie between the two limiting layers above men- 

 tioned. This nomenclature is very objectionable, and indeed 

 has not met with very general acceptance. If limited to the 

 Coelenterata it would be sufficiently expressive and consistent, 

 but when applied to the higher groups of the Metazoa it be- 

 comes utterly impossible of application. To begin with, it is 

 hard to draw any sharp line between the external and internal 

 limiting membranes in the higher Metazoa; in forms where 

 the storaodaeum and proctodaeura are derived from epiblastic 

 invaginations, and form no inconsiderable part of the digestive 

 tract, the confusion becomes complete. In the Isofoda, for 

 instance, nearly the whole of the digestive tract is fori.ied from 

 the storaodaeum and proctodaeum ; thus, according to the Hert- 

 wigs, this clearly epiblastic internal limiting layer would be 

 called endoderm. In the Vertebrate phylum, also, the adult 

 nervous system, clearly derived as it is from the epiblast, 

 would, because it lies between the two limiting layers, come 

 under the name of mesoderm. Moreover, it is an altogether 

 unscientific and confusing method to classify tissues by their 

 position in the adult rather than by their derivation from the 

 primitive germ layers. 



The words ectoderm, mesoderm, endoderm, have become so 

 universally used as the equivalents of epiblast, mesoblast, hy- 

 poblast, that there is very little hope of their being now 

 limited to the group of Coelenterata to which they were origi- 

 nally applied, and this being the case we ought to consider 

 how far the median supporting lamella of that phylum, the 

 Stiitzlamelle or Gallertschichte of German authors, is homolo- 

 gous with the mesoblast of the higher Metazoa before apply- 

 ing the name mesoderm to it. If it is not homologous, but is 

 of a different nature, then some other name than mesoderm 

 should be found for it, otherwise it will be confused with the 

 true mesoblast. 



This opens two questions (1) What do we mean when we 

 speak of a true mesoblast ? (2) What are the characters of the 



