MATURATION OF OVUM IN ECHINUS ESCULRNTUS. 205 



of cells. 2. The sister threads round which the segments split 

 are fused by their ends up to the metakinesis. 3. The 

 monaster stage is short lived, and shows a radial arrange- 

 ment only indistinctly on account of the twisted position of 

 the threads. 4. The end stage of the metakinesis is very 

 prolonged, and has a very special character, in consequence 

 of the fusion of the ends of the threads. 5. A temporary 

 and not understood second longitudinal cleavage of the 

 threads appears in the anaphase. The outstanding feature 

 of the heterotype was considered at first as being the in- 

 complete separation of the two halves of the longitudinally 

 split rods resulting in the formation of ring chromosomes, 

 but the figures may assume very various forms according as 

 the loop is bent, or drawn out so as to obliterate the hollow 

 of it. Again, the rings or their derivatives may be attached 

 to the spindle in different fashion, so that in their resolution 

 different irregular figures emerge. This is shown in the 

 series of diagrams given in the paper of Farmer and Moore, 

 who first clearly pointed out the essential resemblance of 

 the heterotype in plants and animals. The feature described 

 by Flemming, namely, the second longitudinal cleavage 

 found in the anaphase, seems until recently to have had very 

 little significance attributed to it. 



The simplest idea of heterotypical division is that the two 

 halves of the ring-shaped chromosomes are drawn out into 

 U- or V-shaped daughter loops. This simple explanation will 

 not explain many of the figures observed. Farmer (1895), 

 in a study of the phenomena in the lilies, described a 

 double cleavage taking place simultaneously in different 

 planes as the compound chromosome is resolved into its 

 daughter elements. In 1896, along with Moore, he gave an 

 explanation of the phases, which only involved one split, the 

 second being merely apparent. The idea elaborated was 

 that the elliptical ring was bent on itself, applied to the 

 spindle at its apex, and then drawn out to the poles from 

 the point of bending. The original ends were ultimately 

 broken across at the equator. Moore, in his work on 



