THE ACHROMATIC FIGURE 153 



been thought probable that the karyolymph is allowed to react with some 

 cytoplasmic constituent at this time by the disappearance of the nuclear 

 membrane or by alterations in its permeability, the reaction occurring 

 inside or outside the membrane in different cases. ^ 



It seems proper, then, to regard the complete anastral mitotic figure 

 in the metaphase simply as the nucleus in a stage of division, its several 

 chromonemata being doubled and surrounded by condensed matrices 

 and its prophasic karyolymph being organized as the spindle. The 

 division is carried out in a cytoplasmic medium, but the structures con- 

 cerned are primarily those of the nucleus. That mitosis in an ordinary 

 nucleus is in a sense a cooperative undertaking on the part of its several 

 chromosomes is suggested by cases in which each chromosome of the 

 group begins by developing an individual spindle more or less independ- 

 ently of the others. 



Amphiastral Figures. — Achromatic figures of the amphiastral type 

 are characterized by the presence of a system of radiations known as the 

 aster about each pole in the metaphase and a centrosome at the focus of 

 each aster. Such figures are the rule in animals; relatively few cells, 

 notably certain oocytes, are devoid of centrosomes and asters during 

 mitosis. They are also found in certain algae and fungi, as well as in 

 the spermatogenous cells of bryophytes and those vascular plants having 

 motile male gametes. Since the asters are cytoplasmic, the amphiastral 

 figure, unlike the anastral type, is not composed wholly of nuclear 

 materials. 



The centrosome varies widely in structure in different tissues.'* What 

 may be called a "typical" centrosome lies in the cytoplasm (centrosomes 

 rarely occupy the nucleus) and consists of a deeply staining granule 

 known as the centriole together with a surrounding mass of substance 

 called the centrosphere. Either of these elements may be present alone. 

 The centriole may be single, but more commonly it is double as a result 

 of division during the later phases of the previous mitosis. Occasionally 

 there are several centrioles, constituting together a "microcentrum." 

 The centrosphere substance is often fairly abundant in resting cells 

 (Fig. 83). 



In 1887 van Beneden and Boveri observed in Ascaris that the cen- 

 trosome, prior to cell-division, divides to form two daughter centrosomes, 

 which move apart to opposite sides of the cell and form the poles between 

 which the mitotic figure is established; and further, that after cell- 

 division is completed the centrosome included in each daughter cell does 



3 Nothnagel (1916), Tischler (1921-1922). 



* An exhaustive account of centrosomal differentiations is given by Heidenhain 

 (1907). The terminology of the subject has long been in a confused state (see Wilson, 

 1900, 1925). In the present account we have chosen more or less arbitrarily a termi- 

 nology which seems to have found favor with a number of writers. 



