HISTOLOGY OF ASTRAEIDS 117 



partial differentiation into three strata. Towards the base 

 of the column-wall, the middle lamina is absent in places, the 

 calicoblastic layer and the endoderm merging into each 

 other. In such places the appearance is that of one sheet 

 of nucleated protoplasm with an outer granular stratum con- 

 taining large oval nuclei tangentially placed at intervals, which 

 represents the calicoblastic layer, and an inner stratum whose 

 nuclei are smaller but more numerous and placed vertically, 

 which represents the endoderm (fig. 10). 



Since the middle lamina of the Astraeidae is nucleated, 

 formed early in development, and is of the nature of connective 

 tissue, it is comparable to the mesoblast and mesoderm of other 

 animals.^ Bourne in 1887 restricted these terms to denote the 

 intermediate layer of the triploblastica (which he apparently 

 identified with the coelomata), on the view that the middle 

 lamina of Coelenterates was neither embryonic nor ' cellular ', 

 to which he gave a new name, mesoglaea (5, p. 311). This 

 nomenclature was subsequently accepted by most authors — 

 Haddon, van Beneden, Hickson, Ashworth, McMurrich, 

 Duerden — who regarded the nuclei occurring in the middle 

 lamina of Coelenterates as belonging to cells which secondarily 

 migrated into the gelatinous secretum from one or both of the 

 protoplasmic laminae — a view to which my studies on the 

 Madreporaria lend no support. Moseley, von Heider, 

 0. and E. Hertwig, and other earlier zoologists had described 

 the intermediate layer of the Anthozoa under the term 

 mesoderm. This prior usage was resumed in 1895 by Faurot, 

 who, from his comparative study of many Actinian species, 

 disagreed with Bourne in regard to its supposed extra-proto- 

 plasmic formation and structureless consistency and the need 

 for a new terminology. It is also clear from the embryological 



^ Bourne states that ' by mesoblast is meant a layer of undifferentiated 

 cells, developed in the embryo before the differentiation of other organs 

 or tissues from either one or the other or both of the primary germ-layers, 

 the epiblast and hypoblast. By mesoderm and its adjective mesodermic 

 are meant all such tissues in the adult as are clearly derived from the 

 mesoblast ' (5, p. 314). 



