232 Mr Potts, A Note on Vital Staining 



The nematode is surrounded by an elastic cuticle which is con- 

 tinued into the gut at the mouth and anus. Anteriorly it lines the 

 buccal cavity and oesophagus and posteriorly the short rectum. 

 When the animal moults this lining is detached with the external 

 cuticle and retains its shape after detachment. The midgut or 

 intestine consists of a single layer of cells which according to some 

 authorities {Cambridge Natural History, Vol. it. p. 131) is 'coated 

 internally and externally by a layer of cuticle.' In my experience, 

 however, of free-living forms the cells of the midgut do not possess 

 a firm and definite cuticle like that of the oesophagus and rectum 

 but the cell membrane is thin and without any apparent structure. 



The frequent pulsation of the second oesophageal bulb is the 

 means by which the liquid culture medium is pumped into the 

 gut of the nematode. Hence, when the animal is put into a solution 

 of stain this rapidly penetrates into the cavity of the oesophagus 

 and the anterior part of the midgut. It is some time, however, 

 before the whole lumen of the gut is filled with the stain. But 

 none of the neutral red ever penetrates through the external cuticle. 

 The cuticle of the buccal cavity and oesophagus is apparently 

 porous enough to allow the penetration of a certain amount, but 

 the greater part passes through the cell membranes of the mid- 

 gut. When a nematode has been in stain for about six hours it 

 presents a remarkable appearance, the gut being stained pink or 

 purple in its various parts while the remaining tissues are still per- 

 fectly transparent. The stain is taken up by the various scattered 

 granules of the gut-cells and when their capacity for stain is ex- 

 hausted, which does not occur till the experiment has lasted about 

 twelve hours, the neutral red begins to pass through the external 

 membrane of the gut and irregular deposits appear in the genital 

 organs and the lateral fields of the hypodermis. Lastly, the muscle 

 fibres take up the stain, showing a transverse banding of the 

 individual fibres. It does not appear that the slow penetration is 

 due to the resistance of the external membrane of the gut, but 

 merely to the avidity with which the gut granules take up the stain. 



In the midgut cells of Diplogaster these granules fall into two 

 categories: (1) brownish highly refractive granules of various sizes, 

 resistant to most reagents and scattered irregularly through the 

 cell. Their composition is unknown, but they take up a good deal 

 of stain. It is not proposed to deal with them in the present paper. 

 (2) Smaller secretory granules of uniform size forming a peripheral 

 zone round the lumen of the gut, normally colourless, but purple 

 with neutral red. They are especially thickly developed in the first 

 four cells between which the gut cavity is enlarged and often 

 occupied by a bolus of living or dead bacteria. 



There are, however, individuals which do not show the granular 

 zone, but instead the gut-lumen appears to be lined with a structure- 



