66 SPERMATOGENESIS [CH. 



formation it is quite clear that they are in reality double 

 rather than quadruple, being composed of two rods, some- 

 what bent and with their ends in contact, and each deeply 

 constricted in the middle. The constriction corresponds 

 with the point to which the spindle-fibre is attached, and 

 which becomes the apex of the V-shaped chromosome of 

 the anaphase stage, and is seen also in ordinary somatic 

 chromosomes in Lepidosiren. The apparently four-fold con- 

 dition of the chromosomes at this stage is of importance, 

 since it closely resembles the "tetrad" form which at one 

 time was regarded as characteristic of this stage of the 

 developing germ-cell, as will be mentioned below. 



The double chromosomes which thus arrange themselves 

 in the equatorial plate of the spindle are in every case com- 

 posed of two similar halves ; the chromosomes of Lepidosiren 

 differ considerably among themselves in size and shape, but 

 each ring is composed of two similar members, and it can 

 hardly be doubted that the two members of each ring are 

 the same two which paired together in the bouquet stage 

 and which composed the rings in the synizesis that fol- 

 lowed it. On the equatorial plate they are arranged so that 

 one member of each pair is directed towards one pole of the 

 spindle and the other towards the other pole, and the two 

 members of each pair then begin to separate from each other 

 and diverge towards the poles. They remain connected 

 for a time by two strands of chromatin connecting the ends 

 that were in contact, giving a very characteristic appear- 

 ance which is typical of the division of the primary sperma- 

 tocytes, or of the "reducing" division generally, in a large 

 number of animals and plants (PL V, G, H, 7). A'division 

 of this form is known as a heterotype mitosis 1 , and results in 



1 The heterotype or heterotypical form of mitosis was formerly regarded 

 as characteristic of, and confined to, the first maturation division of the 



