LABRAL GLANDS OP SIMOCEPHALUS 210 



there is no distinct, division between these two cells, but the 

 nuclei are placed with their concave sides facing towards each 

 other and in between the two is a very conspicuous and clearly- 

 delined reservoir (PI. 10, figs. 10 and 12). This is apparently 

 formed of a flat plate of transparent coalescing vacuoles of the 

 secretion produced by the gland-cells. 



Neither of these cells possesses an efferent duct as figured 

 by Cunnington, but posteriorly they embrace a separate duct- 

 cell (Pis. 9 and 10, figs. 6, 11, and 12). This cell has the form of 

 a tube opening to the exterior at its posterior end and anteriorly 

 opening into the reservoir of secretion. The lumen of this tube 

 is often flat (PI. 10, fig. 11) especially at its posterior end. The 

 nucleus of this duct-cell stains very lightly and is small compared 

 with that of a gland-cell, although it is slightly larger than that 

 of a nerve- or muscle-cell. The cytoplasm stains very lightly 

 and is not vacuolated. 



In sublimate material there is in the secretion reservoir 

 a granular coagulum which stains faintly blue with Mallory's 

 stain, while in the lumen of the duct-cell it stains red. Pre- 

 sumably the cytoplasm of the duct-cell alters the constitution 

 of the secretion in some way, so that its staining reaction when 

 fixed is changed. A section through the duct-cell at its anterior 

 part shows the secretion in contact with the walls of the tube 

 staining red, while that more centrally placed, which has not yet 

 been acted upon by the duct-cell, still stains blue. The external 

 apertures of the duct-cells form two small slits on the side of 

 the labrum near its tip (PI. 9, fig. 7) where the latter is com- 

 pressed laterally. They are situated a little towards the dorsal 

 surface of the labrum and are ventral to about mid-way 

 between the mandibles and maxillae. 



In other Daphnids studied it was not found possible to obtain 

 preparations sufiiciently well fixed on which to base critical 

 considerations, but it is evident that the same ground-plan 

 underlay all the cases studied. In Chirocephalus, however, 

 the results obtained are very good and agreed comparatively 

 well with Claus's (4) figure for Branchipus. The proximal 

 group is very scattered and ill defined. Its cells do not all line 



NO. 262 Q 



