Mr Potts, A Note on Vital Staining 



233 



less highly refractive layer which stains a brownish-red. Some 

 Diplogasters show the granules anteriorly in the gut and the struc- 

 tureless layer posteriorly, but never both together. The explanation 

 of this distribution is given by observing that the granules break 

 down and form the substance of the structureless layer. This is a 

 change which can be watched, beginning posteriorly and working 

 slowly forward until the whole of the midgut is lined by the 

 substance. 



The structureless layer is not a firm cuticle as is shown by the 

 fact that when a stained Diplogaster is compressed under a cover- 



Diagrammatic figure of Diplogaster 

 sp. to show staining of midgut mth 

 neutral red after about six hours. In 

 the specimen figured the individual is 

 in an intermediate state, the granules 

 being well develoised anteriorly but 

 have broken down posteriorly to form 

 the secretion. The cell boundaries and 

 nuclei of the midgut are omitted. 



/. oesophagus. 



g. granules surrounding lumen of 

 midgut. 



s. secretion formed by breaking 

 down of these. 



rn. midgut \vith larger resistant 

 granules. 



o. ovary. 



slip to such an extent that the contents of the body are squeezed 

 out through the anus, as the gut lining passes through the narrow 

 aperture it changes its form and flows like a plastic material. 

 Moreover the lining of the midgut does not appear as a definite 

 detached layer in moulting or after the death and disintegration 

 of the animal, as is the case in the cuticle of the oesophagus and 

 rectum. * 



That the granules and the substance they produce are not arti- 

 facts or products of degeneration is shown by the fact that they 



