THE SALIVARY APPARATUS 121 



arranged around a central lumen, which is 'constituted by the intra-acinar 

 duct. Outside the cells there is a well-defined basement membrane. At 

 times small secondary acini or bifurcations of one or other of the three 

 glands are met with. 



The lateral glands, which are of what Christophers terms the ' granu- 

 lar ' type, are generally almost filled with a granular secretion, which 

 compresses the protoplasm and the nucleus of the cell towards the outer 

 border. In the fresh condition the secretion is clear and refractile, 

 while in sections it appears as a coarse reticulum, due, as pointed out by 

 Christophers, to a precipitation of the secretion by the fixative. It does 

 not stain with eosin, and only faintly with haematoxylin. There is some 

 reason to believe that the portion of the gland adjacent to the junction 

 of the three ducts is different to the rest, for Grassi notes that its secre- 

 tion is more refractile, while Christophers states that the cells in this 

 situation are smaller, and the lumen of the gland correspondingly 

 restricted. In some species of Culex the reverse is the case, the cells 

 in this region being cylindrical ; there is, however, a good deal of differ- 

 ence in different species and individuals in the appearance of the 

 glands. 



The central glands are of what Christophers terms of the ' colloid ' 

 type. The cells are not so much compressed, and contain more pro- 

 toplasm than those of the lateral glands. Unlike that of the latter, 

 the secretion stains well with eosin and also with haematoxylin ; it is 

 less refractile in the fresh condition. 



The appearance of the intra-acinar duct differs a little in Culex and 

 Anopheles. In the former it is of the same width throughout, and 

 terminates near the blind end of the gland, while in Anopheles it termi- 

 nates in an indefinite manner about the middle, so that the upper 

 portion of the gland is expanded and forms a reservoir for the storage 

 of saliva. The main ducts give off a few small lateral branches, which 

 end blindly. Under a high magnification minute pores can be seen in 

 the chitinous wall of the ducts. 



Little or no change takes place in the glands as a result of feeding ; 

 it is probable that only a small amount of saliva is passed down at each 

 bite, and that it is very rapidly replaced by the secreting cells. 



In newly hatched and unfed mosquitoes the cells of the gland are much 

 more regular, and the nucleus is little if at all compressed by the 

 secretion. In this condition the nucleus stains very deeply and is 

 placed centrally. The protoplasm distal to the nucleus contains many 

 very fine granules and one or two vacuoles. 

 16 



