MORPHOLOGICAL COMPOSITION OF THE NUCLEUS 30I 



conditions, i.e. with the number of chromosomes that enter the 

 nucleus. Neither can we assume that it is due merely to the total 

 mass of the chromatin in each case ; for this varies in different nuclei 

 of the same species, or even in the nucleus of the same cell at dif- 

 ferent periods (as in the egg-cell), yet the same number of chromo- 

 somes is characteristic of all. Indeed, we seek in vain for an analogy 

 to these phenomena and can only admit our entire inability to explain 

 them. No phenomena in the history of the cell more clearly indicate 

 the existence of a morphological organization which, though resting 

 upon, is not to be confounded with, the chemical and molecular 

 structure that underlies it ; and this remains true even though we are 

 wholly ignorant what that organization is. 



{b) Composition of the Chromosomes. — We owe to Roux^ the first 

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

 thread, consist of successive regions or elements that are qualitatively 

 different (p. 244). 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 estabhshed (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 unmistakably to the conclu- 

 sion that these granules are perhaps to be regarded as independent 

 morphological elements of a lower grade than the chromosomes. 

 That they are not artifacts 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 morphological nature is, 

 however, even more difficult than in the case of the chromosomes ; 

 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 undetermined. 

 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 chro- 

 matin-discs arranged in a linear series (Van Beneden's figures of the 

 same object show at most five) which finally fuse to form an appar- 

 ently homogeneous body. In the chromosomes of the germ-nuclei 

 the number is at least double this (Van Beneden). Their number has 

 been more carefully followed out in the spermatogenesis of the same 

 animal (variety bivalejis) by Brauer. At the time the chromatin-grains 



^ Bedeutung der Kerntheilungsjiguren, 1 883, p. 15. 



