102 RECORD OF CURRENT RESEARCHES RELATING TO 



usually ectoderm, mesoderm, and eudoderm, since they probably cor- 

 respond with the layers known by these names in the higher animals. 

 But as this question is more or less siih judice, Schulze prefers to speak 

 of "outer cell layer" (aussere Zellenschicht), "connective layer" 

 (Bindesubstanzschicht), and " collar - cell layer " (Kragenzellen- 

 Bchicht). 



The outer cell layer consists of a single layer of flattened polygonal 

 cells with rounded nuclei ; it can easily be made out in all the lacunte 

 and canals with the exception of the ciliated chambers, and exists also 

 on the external surface, where, however, its existence is more difficult 

 to demonstrate. In many examples a true cuticle can be separated 

 from the outer surface as a delicate, hyaline, elastic lamella. 



The connective layer consists, in all parts where ciliated chambers 

 do not occur, of a hyaline, gelatinous ground-substance, with anasto- 

 mosing connective-tissue corpuscles embedded in it. In the tissue 

 immediately surrounding the ciliated chambers the ground-substance 

 contains a great number of rounded, highly refractive granules, which 

 give to these portions of the sponge tissue, a white, opaque appearance, 

 by reflected light. Some of the connective-tissue corpuscles contain 

 pigment granules. Amongst the ordinary corpuscles, rounded cells 

 without j)seudoj)odial processes are to be found, containing, in addition 

 to a nucleus, highly refractive spheroids, probably of a fatty or 

 amyloid nature ; probably these are reserve materials, like the some- 

 what similar bodies found in Chondrosia and Aplysina* 



Contractile fibre-cells — muscle-cells of other authors — occur in the 

 mesoderm, especially in the outer layer, around the canals, and in the 

 Bphincter-like membranes of the oscula. Spindle-shaped fibres were 

 also found gathered into definite bundles, and usually running parallel 

 to the canals : probably these are antagonistic to the circular fibres. 



As in Aphjsina, Spongelia, &c., the mesoderm bounding the cavities 

 in which the generative products occur is modified so as to form a 

 single layer of flat endothelial cells. 



In the horny fibres, developed entirely in the mesoblast, Schulze 

 distinguishes an axial cord of soft consistency, and a thick, laminated, 

 highly refractive cortex, the layers of which are not composed of 

 longitudinal fibrillfe, but are homogeneous and structureless. As to 

 the origin of the skeleton, he states that " the horny fibre is a cuticular 

 secretion of peculiarly modified connective-tissue cells, the spongo- 

 hlasts," which envelope the fibres as a single layer of epithelium-like 

 cells of variable size. It is probable that when new fibres are to be 

 formed, ordinary mesoderm cells become somewhat altered in shape, 

 and arranged in strings ; that there is then formed in the axis of the 

 string, partly by secretion from the cell, partly by modification of the 

 mesodermal ground-substance, the soft axial cord ; and that then, the 

 cells, acquiring the characteristic form of spongoblasts, secrete, layer 

 by layer, the cortical substance. The outer free ends of the principal 

 fibres grow, probably, in virtue of a cap of irregular mesoderm cells 

 which invest them : in growing, it is probable that they include in their 



* For ibis and other points, cf. Schulze on Aphjsina and Spongelia, this 

 Journal, ii. (1879) p. 434. 



