280 W. G. VanName — Embryology of Eustylochus. 



arms of which have come together and fused. As in the form first 

 described, a transverse division of the rod at its middle point finally 

 separates the derived segments. In the latter form, however, they are 

 joined by both ends, in the former by but one end of each. This 

 process is shown diagrammatically in Fig. 42. Actual examples in 

 different stages appear in Figs. 9, 10 and 11. 



Both these processes were recognized by Francotte but described 

 with especial care and fullness by Van der Stricht (28). Klinckow- 

 strom figured them but gave little explanation. We often — indeed 

 I might say generally — find diflFerent stages of both of these processes 

 going on in different chromosomes of the same spindle. Of each 

 there are many minor modifications, so that a great variety of forms 

 results. Most of them are due to the tendency of the derived seg- 

 ments to thicken at certain points, especially at the middle (where, 

 as I shall describe later, they are probably destined to divide in the 

 second polar spindle) and at the ends, where they form knobs. These 

 thickenings often become conspicuous long before there is any sign of 

 separation at these points, or even in the ring stage. The result, 

 when it occurs early, is that the ring is thickened at four points, 

 giving the appearance of a tetrad group connected by narrower 

 commissures. Such figures are mentioned by Francotte. It is how- 

 ever in these thickened places, and not between them, that the 

 chromosome finally divides. 



By far the most interesting modification is one which gives us 

 some suggestion in regard to the distribution of the chromatin. 

 The actual partition of the chromosome in both the methods 

 above described is transverse to its long axis. The important 

 question is whether this partition is transverse to the original long 

 axis of the beaded segments found in the early stages of spindle 

 formation, in other words, whether we have an equal or a reducing 

 division of the chromatin. The modification which gives us a hint 

 in regard to this important matter is shown in Fig. 43, examples of 

 it also in Fig. 10. 



This diagram (Fig. 43) shows not successive stages in the history 

 of a chromosome as the other two diagrams, but different degrees of 

 modification from the type shown in Fig. 42. As will be understood 

 from the figures, in its extreme form it results in a cross-shaped 

 chromosome. Such forms are described but not explained by Van 

 der Stricht. The interesting point about this form is that it would 

 hardly be possible to produce it unless the cleavage of the chromo- 

 some which produces the ring is actually a longitudinal one. Here 



