312 GILBERT C. BOURNE. 



the name mesoderm for the median layer of the Coelenterate 

 body. But, as the Hertwigs have pointed out very clearly, 

 from the time that the primary germinal layers, the epiblast 

 and hypoblast, of the higher Metazoa were first compared 

 and homologised with the ectoderm and endoderm of the 

 Coelenterata, there has been an increasing tendency to use the 

 names ectoderm and endoderm as the equivalents of epiblast 

 and hypoblast ; and this is especially the case among German 

 authors, with whom the use of the names epiblast, mesoblast, 

 hypoblast for the germ layers of the embryo, has not found 

 general acceptance. It followed that the name mesoderm 

 came to be used in the same sense, or very nearly so, as meso- 

 blast, instances being numerous among German authors, and 

 not infrequent even among English authors, where the meso- 

 derm of the germ or embryo is spoken of. The difficulty 

 arising from the identical use of these two names was appre- 

 ciated by the Hertwigs and by F. E. Schulze, who treated 

 the subject at some length, each in their own way. F. E. 

 Schulze asks the very pertinent question whether the name 

 mesoderm can only be used in those cases in which a special 

 layer of cells arises early, that is, before the development of 

 tissues and organs, as a special germ layer; or whether one 

 can speak of a mesoderm when a differentiation of a special 

 middle layer of tissue from the outer or inner epithelial layer 

 arises later and without the formation of a special germ layer. 

 He concludes by drawing a distinction between triple- walled 

 animals, such as the Coelenterata, and those which have three 

 germinal layers (viz. the higher Metazoa (Triploblastica) but 

 not the Coelenterata), admitting at the same time that the 

 Coelenterates have not a mesoderm in the sense of a distinct 

 layer of cells derived from either or both of the two primary 

 germ layers before the latter show any differentiation into 

 tissues or organs. He speaks of them as being " dreischich- 

 tige^' but " zweiblattrige." The Hertwigs, in dealing with the 

 difficulty, proposed to limit the use of the words ectoblast and 

 entoblast (i. e. epiblast and hypoblast) to the germinal layers 

 of the embryo, and to use the names ectoderm and endoderm 



