212 FILARIA BANCROFTI [CH. 



slightly from the anus backwards, and bears the above-men- 

 tioned papillae. 



Sixth Stage. The movements now become more active. 

 The lips of the mouth are closely pressed together in the form 

 of a cone and minute horny papillae are present. The worm 

 now measures about 1*5 mm. by 0*03 mm. 



Until 1900 it was supposed that this was the final stage in 

 the development, and that the worm now required to escape 

 into some water, in which it swam about until it was taken up 

 by man in drinking water. In that year Low shewed that 

 there was yet another stage. 



Seventh Stage. When the filariae have reached their highest 

 development in the thoracic muscles they leave that tissue 

 and begin to travel forwards, probably as a result of some 

 chemiotactic attraction. They pass into the loose cellular 

 tissue in the prothorax near the salivary glands, then force 

 their way through the neck and coil themselves up in the 

 loose connective tissue just below the cephalic ganglion and 

 salivary sac. Finally, they pass into the proboscis of the 

 mosquito by making an independent passage through Button's 

 membrane at the base of the labrum, and pushing forward 

 between the labrum and hypopharynx amongst the stylets. 

 Here the worms are found stretched along the length of the 

 proboscis, head foremost Two worms, probably male and 

 female, are nearly always found together. 



In this stage the worm is usually about i mm. in length 

 and 0*025 mm - m breadth. It tapers slightly towards each 

 extremity, and at the anterior end the cuticle is thickened in 

 places to form a few small papillae arranged round the terminal 

 mouth. The posterior end, which is rounded off, is provided 

 with three papillae almost at right angles to the axis of the 

 body of the worm. The alimentary canal is straight and shews 

 no marked differentiation into oesophagus and intestine. 

 Towards the anterior end, 0*14 mm. from the extremity, 

 there is an indication of the presence of an orifice, towards 

 which the genital duct is seen to bend. 



These young filariae remain in the proboscis of the mosquito 

 until it feeds on some host. The worms then escape and may be 



