histology of astraeids 115 



General Considerations. 



In the tissues of the Astraeidae, celMimits cannot be 

 discerned, the nuclei lying immersed in the general protoplasm. 

 This is particularly the case in the surface ectoderm, inner 

 lining of stomodaeum, and in mesenterial filament. While 

 in the endoderm, nuclei tend to lie between vacuoles, in the 

 middle lamina nuclei are few and the protoplasmic areas con- 

 taining them are not definitely circumscribed but appear to 

 be organically connected together by means of their radiating 

 strands. Sections of 4 /li and 6 fx thicknesses treated with silver 

 nitrate failed to show any cell-limits, nor is a cellular structure 

 seen in sections cut in gelatin with Aschoff's COg freezing 

 microtome, nor again in celloidin sections of polyps. Duerden 

 observed that in S i d e r e n t s e a radians the endoderm 

 of the wall of the polyp lining the uppermost parts of the 

 skeleton is ' a syncitium showing no signs of cellular divisions ', 

 and that the calicoblastic layer ' in the growing areas of the 

 skeleton shows no evidence of cell limitations ' (9, pp. 30, 31). 

 Gardiner, too, could not find definite cell outlines in Coeno- 

 psammia and Flabellum (13 and 14). Such outlines, so 

 frequently represented in figures of the ectoderm and endoderm 

 of the Anthozoa, are doubtless conventional and arbitrary. 



The products of teasing of the tissues, whether before or 

 after maceration, cannot be regarded as separated units of 

 structure or ' cells ', but are really bits of protoplasm inevitably 

 torn apart with the nuclei in the mechanical process of teasing. 

 Hence, it is generally found that such pieces of protoplasm 

 possess neither regular nor uniform contour and are sometimes 

 torn apart with fibres of the middle lamina. If an appearance 

 of cellular strands is noticeable in some preserved tissues, it is 

 due to the shrinkage of protoplasm around the nuclei, which 

 probably act as centres of force, H. V. Wilson regarded the 

 endodermal mass of the solid planula stage of Manic in a 

 areolata as a ' plasmodium which was subsequently broken 

 up into cells ' (37, p. 200) ; he regarded the earlier blastosphore 

 as composed of cells, although their inner ends were ' not 



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