1 .I 



INTRODUCTION TO CYTOLOGY 



Special emphasis was laid upon this interpretation by Marechal (1904, 

 1907) as a result of his studies on the growth stage of animal oocytes. 

 At this period in the development of the ovum the chromosomes assume 

 a finely branched form (Fig. 86, C, D) and their ordinary staining capacity 

 is lost completely. Although the chromatic fluid may flow from the 

 reticulum to. the nucleolus and vice versa, and may periodically undergo 

 chemical changes which radically alter its staining reactions, the achro- 

 matic chromosomal substratum nevertheless maintains an uninterrupted 

 structural continuity. Such a transfer of the basichromatic material 

 from the persistent reticulum to the nucleolus during the telophase, and 

 to the reticulum again during the succeeding prophase, has also been 



FIG. 55. Some evidences for chromosome individuality. 



A, chromosomal vesicles in Brachystola magna; x-chromosome in vesicle at right. 

 (After Sutton, 1902.) B, chromosomal vesicles in Fundulus embryo. X c. 1800. (After 

 Richards, 1917.) C, chromomere vesicle (c) on chromosome of Chorthippus. X 1500. 

 (After Wenrich, 1917.) D, prochromosomes in Pinguicula. X 4200. 



observed by Strasburger (1907) and Berghs (1909) in the somatic nuclei 

 of Marsilia (Fig. 17, E}. The chromosome, as Marechal urges, is not 

 simply a mass of chromatin, but rather "a structure periodically chro- 

 matic;" hence the disappearance of stainable substance does not signify 

 the loss of structural continuity on the part of the chromosome. 



The "chromosomal vesicles" (Fig. 55, A, B) observed by certain 

 investigators constitute valuable evidence in this connection. In the 

 spermatogonia of the grasshopper, Phrynotettix, for example, Wenrich 

 1916) has shown that each of the alveolizing chromosomes forms its 

 own vesicle about it at telophase, the several vesicles joining to form a 

 common nucleus. In some cases the boundaries between the vesicles do 

 not entirely disappear during the resting stages, and at the next prophase 



