MORPHOLOGICAL COMPOSITION OF THE XUCLEUS 221 



the highest significance^ that these more elementary units group 

 themselves into definite aggregates of a higher order that show a 

 certain degree of persistent individual existence. It may be said 

 that the tendency to assume such a grouping is merely a question 

 of nuclear dynamics, and is due to a "formative force" innate in 

 the chromatin-substance. This is undoubtedly true ; but it is only 

 another form of expression for the facts, though one that avoids the 

 use of the quasi-metaphysical term "individual." Whether a chro- 

 mosome that emerges from the resting nucleus is individually the 

 same as one that entered into it can only be determined when we 

 know whether it consists of the same group of chromatin-granules 

 or other elementary bodies. It must not be forgotten, however, that 

 in the case of the egg the chromosomes may persist without loss of 

 their boundaries from one division to another, since no reticulum is 

 formed (cf. p. 193). 



{b) Compositio)i of the Cliromosomes. — We owe to Roux^ the first 

 clear formulation of the view that the chromosomes, or the chro- 

 matin-thread, consist of successive regions or elements that are 

 qualitatively different (p. 183). This hypothesis, which has been 

 accepted by Weismann, Strasburger, and a number of others, lends 

 a peculiar interest to the morphological composition of the chromatic 

 substance. The facts are now well established (i) that in a large 

 number of cases the chromatin-thread consists of a series of granules 

 (chromomeres) embedded in and held together by the linin-substance, 

 {2) that the splitting of the chromosomes is caused by the division 

 of these more elementary bodies, (3) that the chromatin-grains may 

 divide at a time when the spireme is only just beginning to emerge 

 from the reticulum of the resting nucleus. These facts point unmis- 

 takably to the conclusion that these granules are perhaps to be re- 

 garded as independent morphological elements of a lower grade than 

 the chromosomes. That they are not artefacts or coagulation-products 

 is proved by their uniform size and regular arrangement in the thread, 

 especially when the thread is split. A decisive test of their morpholog- 

 ical nature is, however, even more difficult than in the case of the chro- 

 mosomes ; for the chromatin-grains often become apparently fused 

 together so that the chromatin-thread appears perfectly homogeneous, 

 and whether they lose their individuality in this close union is unde- 

 termined. Observations on their number are still very scanty, but 

 they point to some very interesting conclusions. In Boveri's figures 

 of the egg-maturation of Ascaris each element of the tetrad consists 

 of six chromatin-disks arranged in a linear series (Van Beneden's 

 figures of the same object show at most five) which finally fuse to 



1 Bedeuliing der Kertitheilungsfiguren^ 1883, p. 15. 



