118 GEORGE MATTHAI 



studies of Joiirdan on Actinia equina and Balano- 

 phyllia regia, of E. B. Wilson on Renilla, and of 

 H. V. Wilson on ^I a n i c i n a a r e o 1 a t a , that the middle 

 lamina appears early in development — in the solid planula 

 stage. I have found this to be the case in the solid embryos 

 of Fa via fragum. Jourdan distinguishes between a 

 ' membrana propria ' and a granular mass ; while the origin of 

 the former was uncertain, the latter was said to be formed 

 by the severance and fusion of the inner ends of the ectoderm 

 cells of the body-wall, which subsequently become fibrous.^ 

 E. B. Wilson, wdio made a more or less similar distinction, 

 also found that the middle lamina of the body-wall was 

 formed by the separation and fusion of the swollen inner ends 

 of ectoderm cells, though he somewhat arbitrarily termed the 

 process ' a pecuhar form of cuticular secretion ' (36, p. 759). 

 Bourne's account of the formation of the middle lamina in 

 H e 1 i o p o r a (6) is not different from those of J ourdan and 

 E. B. Wilson. 



Although a discussion of the highly controversial subject 

 of the history and homology of the germ-layers of the Metazoa 

 does not lie within the scope of this paper, it will be seen 

 from the foregoing account that there was not adequate 

 reason for withholding the apphcation of the term mesoderm 

 to the middle lamina of the Anthozoa. 



Bibliography. 



1. Allman, G. J.^ — " On the Anatomy and Phjsiology of Cordylophora ", 



• Phil. Trans.', cxliii, 1853. 

 2. "On the Structure and Development of Myriothela ", ibid., 



clxv, p. 549, 1875. 

 3. Ash worth, J. H. — " The Structure of Xenia Hieksoni, nov. sp., with 



some Observations on Heteroxenia Elizabethae, KoUicker ", 



' Quart. Journ. Micr. Sci.', xlii, ]i. 245, 1899. 



1 The mode of formation of the middle lamina of certain Alcyonarians 

 described by Kowalevsky and Marion (23) is essentially similar to that 

 of Jourdan, but in their subsequent discussion they, however, came to the 

 conclusion that the middle lamina of Coelenterates was not homologous 

 with the mesoderm of Coelomates. 



