210 INTRODUCTION TO CYTOLOGY 



being used in their formation. In the opinion of Miss Merriman(1913) 

 the chromatic bodies observed by the above workers are not true chromo- 

 somes, but are rather more indefinite chromatic aggregations which are 

 variable in number and appearance, and which are irregularly pulled 

 apart as mitosis proceeds. She finds here "no evidence throughout the 

 karyokinesis of an equational division of autonomous bodies." 



In Zygnema both Escoyez (1907) and van Wisselingh (1914) find that 

 the reticulum, and not the nucleolus, gives rise to all the chromosomes. 

 Although the nucleolus furnishes no morphological element, chromatic 

 material may flow from it to the chromosomes as they develop from the 

 reticulum. Much the same condition is found in Marsilia (Strasburger 

 1907; Berghs 1909). Strasburger points out that in the somatic nuclei 

 (in the cells of the root and the young prothallium) most of the chromatic 

 substance is held in the nucleolus during the resting stages (Fig. 17, E), 

 and that the material of the reticular framework, which is very delicate, 

 is to be regarded as the substance of importance in heredity. Berghs 

 shows that the nucleolus consists of an achromatic substratum which 

 appears independently of the reticulum in the telophase and soon becomes 

 impregnated with chromatic material transferred to it from the chromo- 

 somes. In the next prophase the chromatic material flows back to the 

 delicate reticulum, from which the chromosomes gradually develop. As 

 the chromosomes increase in distinctness the nucleolus becomes paler, 

 and when the nuclear membrane breaks down the nucleolus dissolves in 

 the protoplasmic liquid. It is therefore clear that in Marsilia the nu- 

 cleolus is not a mere aggregation of the chromosomes of the telophase, as 

 might at first be supposed. The chromosomes arise from the reticulum 

 as usual, and not from the nucleolus as reported for Spirogyra. In these 

 observations we have additional evidence favoring the view of Haecker, 

 Boveri, Marechal, and others (see Chapter VIII) that it is the achromatic 

 substratum of the chromosome, and not the chromatic substance which 

 it carries, that should be regarded as the persistent structural unity 

 representing the basis of inheritance. 



Amitosis. — In amitotic or direct nuclear division the nucleus simply 

 constricts and separates into two portions while in the " resting" condi- 

 tion, no condensed chromosomes, centrosomes, spindle, or asters being 

 formed. As a general rule such a division of the nucleus is not followed 

 by a division of the cell ; cells with two or more nuclei therefore commonly 

 result. As examples may be cited the tapetal cells in the anthers of 

 angiosperms, the internodal cells of Char a (Fig. 75) (Johow 1881), and 

 certain glandular cells of animals. The presence of more than one 

 nucleus cannot by itself be regarded as evidence that amitosis has 

 occurred, however. Amitosis appears to be of rather frequent occurrence 

 among the lower organisms, some of which show other methods of divi- 

 sion also. For example, amitosis occurs regularly in budding yeasts, 



