SPERMATOZOA OF THE FLEA AND BLOW-FLY. 315 



3. A fine filament winding as a spiral inside the body of the 

 spermatozoon. This is very difficult to see or photograph and is 

 visible only in some of the individuals. 



Of this fine filament Mr. Nelson writes : " The finer structure 

 shown in your photomicrograph is a true spiral which vin- 

 doubtedly is inside the thread-shaped body. There seem to be 

 five or five and a half turns of this small spiral to one undulation 

 of the exterior ribbon." 



I have also examined the spermatozoa of the blow-fly 

 obtained from the spermatheca of a female. They are much 

 smaller than those of a flea. There are two forms which do not 

 differ much in length, which is about ^tJ^ in. (0'12 mm.), but one 

 is very much thicker than the other ; indeed, this other is possessed 

 of such extreme tenuity that it reminds me of Euclid's definition 

 of a line, i.e. " length without breadth." 



The heads are pointed and the bodies, like those of the flea, are 

 composed of three parts which can be seen only where they seem 

 to have separated accidentally. The two filaments which wind in 

 the spermatozoon of the flea do not wind in that of the blow-fly. 



This dimorphism in spermatozoa when considered in conjunc- 

 tion with the well-established dimorphism in ova seems to indicate 

 that sex goes much deeper into the act of generation than is 

 generally supposed, that in fact a male spermatozoon may be 

 attracted by a female ovum or that a female spermatozoon may 

 unite itself to a male ovum. This is a problem for a definite 

 solution of which scientists will pi-obably require a much more 

 perfect microscope than those which we possess to-day. 



Description of Plate 11. 



Fig, 1. Shows three distal ends indicated by their whip-like 

 filaments. The bulb-like ending of the large form 

 is easily distinguished from the ends of the two 

 smaller forms. The undulating filament is also well 

 seen, x 1500. 



