208 



FILTER-BED NEMAS 



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Chromosomes. Zygosis. Using the best modern instruments, under favorable 

 conditions, it is possible to see the spermatozoa of Plectus cirratus in living 

 Fig. 9. Plectus specimens of the right age and 

 cirratus, which 

 like many other 

 species of Plectus 

 is syngonic. An 

 egg is seen in 2nd 

 maturation divi- 

 sion, chrsm (6 + 6). 

 SHiut 



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condition. The most suitable 

 age for such observations ap- 

 pears to be that at Avhich the 

 spermatozoa are about to enter 

 the uterus, the stage when the 

 sexual organs appear quite sim- 

 ple, and the uterus has the form 

 of a rather narrow empty tube. 

 On searching for spermatozoa or 

 spermatocytes near the large or 

 proximal end of the ovary it 

 will usually be possible to find 

 them without examining many 

 specimens. It may be necessary 

 to view the specimens at a par- 

 ticular angle, and it may require 

 a number of trials to secure a 

 favorable specimen. Fixation 

 with hot corrosive sublimate, 

 followed by an acid carmine 

 stain and long destaining, de- 

 monstrates the presence of sper- 

 matozoa at almost any stage 

 after they are formed, up to the 

 time when all have 

 been utilized. Using 

 such material, I have 

 been able to trace the 

 spermatozoon into the 

 egg, and show that the 

 spermatozoon displays 

 about six chromosomes 

 at a time when its two 

 centrosomes appear on 

 the side next the egg- 

 nucleus, which under 

 its influence divides, 

 and forms two distinct 

 groups of chromosomes 

 consisting of six each. 

 Indefinite bodies are 

 formed in the vicinity 

 of the egg nucleus which 

 appear to have the sig- 

 nificance of polar bodies; see Fig. 9, where one is show r n faintty at corp plr (/.!. 

 Syngonic. My observations leave me without the slightest doubt that this 

 Plectus is syngonic, and that the spermatozoa are functional. Rather casual 

 observations I have made on other species of Plectus prove that syngonism is 



