Artificial Parthenogenesis in Thalassema Mellita 125 



by Thalassema, I have not been able to arrive at a similar inter- 

 pretation for the eggs which I have studied. It should also be 

 mentioned in this connection that Hunter ('04) gives an account 

 of the origin of the cleavage asters for the parthenogenetic eggs of 

 Arbacia which is in harmony with Wilson's view. In a brief note 

 he merely makes the statement that "a small aster with central 

 dark body (parthenocentrosome) appears in contact with exte- 

 rior of nuclear membrane. This divides to form the amphiaster" 

 (p. 214). 



The question of the orgin of the cleavage centrosomes of Thalas- 

 sema will be referred to in a later section of this paper, but I 

 have little doubt that they are new formations. There is not the 

 least indication of a continuity between the egg center and the 

 cleavage centers, the former totally disappearing at the close of 

 maturation, and lam not inclined to accept the view that the egg 

 center persists, without at least some evidence of the fact. 



3 The Cleavage Stages 



The completion of the first cleavage takes place entirely in 

 accordance with Griffin's description of the process in the fertilized 

 egg, except that only 12 chromosomes (the reduced number) are 

 derived from the spireme of the nucleus. 



After the stage drawn in Fig. 27, in which the nucleus is seen 

 somewhat drawn out toward the asters, the nucleus elongates 

 still further and becomes fusiform. As the membrane disappears 

 at the poles, the inner rays of the asters invade the nuclear area 

 and form the spindle, some of the libers appearing coarser and 

 wavy and attached to the chromosomes; these are the so-called 

 "traction fibers." With the development of the rays and spindle, 

 a centrosphere surrounding each centrosome appears, and, as it 

 rapidly increases in size, it becomes reticulated (Fig. 28). At the 

 time of metaphase the centrosomes divide and pass toward the 

 outer side of the spheres. 



Twelve rod-like chromosomes are derived from the spireme 

 thread, which is left in the equatorial plane of the spindle after 

 the disappearance of the nuclear membrane. Their number, 

 which can be readily determined in sections of the equatorial 



