xv NEMATODA 439 



The male cells of Nematoda are not spermatozoa but small 

 amoeboid cells with large nuclei. By carefully opening mature 

 specimens of Ascaris megalocephala and unravelling the genital tubes 

 so gently as not to break or injure them, by then carefully dividing 

 these tubes into short lengths which are numbered so as to show 

 the position of each portion in the tube from which it was taken, and 

 by preserving these portions in separate bottles, material may be 

 obtained for an exhaustive study of the maturation of male and 

 female cells, and it was in this way that Hertwig in 1890 obtained 

 evidence as to the nature of the polar bodies. 



It is a most remarkable fact that there are two races of Ascaris 

 megalocephala, in one of which there are four chromosomes in the 

 oogonia and spermatogonia, and two chromosomes in the ripe eggs 

 and male cells ; and in the other of which there are two chromo- 

 somes in the oogonia and spermatogonia, and one chromosome only 

 in the nucleus of the ripe eggs and male cells. The first variety is 

 called bivalens, the second monovalens. The genital cells at any 

 given cross section of the genital tube are all in the same stage of 

 development. 



The most exhaustive studies of the development of the fertilized 

 egg have been made by Zur Strassen (1896) and by Boveri (1899), who 

 observed the development up to about the 200 -cell stage, also by 

 the former's pupil Miiller (1903), who endeavoured to carry on the 

 analysis of the development up till the time when the young worm 

 was hatched. 



Zur Strassen's method of making whole mounts of the eggs and 

 embryos was as follows. The eggs were fixed by being placed for 

 twenty-four hours in a mixture of four parts of 95 per cent alcohol 

 and one part glacial acetic acid ; they were then stained for 

 twenty -four hours in Grenadier's hydrochloric acid carmine; the 

 excess of stain was removed by immersion for forty-five minutes in 

 95 per cent alcohol to which 1 per cent of hydrochloric acid had 

 been added. They were then washed in 95 per cent alcohol. Some 

 glycerine was added to this, and by allowing the alcohol to evaporate 

 gradually the eggs were got into a solution of pure glycerine. The 

 eggs were examined in this medium, and by gently pressing on the 

 coverslip they could be rolled into various positions and examined 

 from all sides. Boveri recommends similar methods, but he also 

 employed picro-acetic acid, made by adding to a concentrated solution 

 of picric acid in water, two volumes of water and 1 per cent of 

 glacial acetic acid. 



The segmentation of the egg of Ascaris megalocephala is by far 

 the most specialized development that has ever been described. 

 Many workers besides Zur Strassen have examined it, and a nomen- 

 clature has been gradually agreed on, which will be employed here. 

 This nomenclature was not fully developed when Zur Strassen wrote 

 his long descriptive paper, but it is easy to translate the nomenclature 

 which he there employs into the more modern nomenclature. 



