562 A. C. WALTON 



nuclei, their intermingling is possible at any time after the flat- 

 tened membranes between the two pronuclei have broken down, 

 and in case the cytoplasm of the sex cells is a bearer of any hered- 

 itary characteristics, these could intermingle at any time after 

 the sperm enters the egg and loses its separate identity. 



4. Early cleavage 



The first cleavage spindle (fig. 64) arises immediately after 

 the juxtaposition, or the fusion, of the two pronuclei. When a 

 'fusion nucleus' (fig. 63) is produced, the process is similar to 

 the formation of any mitotic figure; the chromosomes arrange 

 themselves in the equatorial plate (fig. 64) and proceed to divide 

 in the usual manner. The more common process, in which the 

 two pronuclei do not fuse until the spindle is formed, is some- 

 what different. The spindle fibers arise upon the disappearance 

 of the portions of the nuclear membranes which are contiguous 

 with each other and those w^hich face the centrosomes. The 

 chromosomes arrange themselves into a single equatorial plate 

 (similar to the one shown in figure 64), but owing to the mor- 

 phological similarity of the chromosomes, it is not possible to 

 locate the male and female groups in definite portions of the 

 spindle. 



The close association of the chromosomes in fixed material 

 often gives the appearance of division en masse (fig. 65), each 

 daughter group receiving half of the chromatic material. Direct 

 evidence as to the deportment of the individual chromosomes 

 during this division is lacking, but some indirect evidence seems 

 to suggest that the plane of separation coincides with the long 

 axis of the chromosome, for in some cases the daughter cells 

 exhibit chromosomes smaller than those of the mother nucleus, 

 but apparently of the same number and form. This could be 

 the case only when the division was longitudinal. A transverse 

 constriction, still evident in the daughter chromosomes, is prob- 

 ably the Querkerbe. Other daughter cells, both those result- 

 ing from the first division, and those from later divisions of the 

 soma cells, show a curious behavior that may afford an explana- 



