120 



INTRODUCTION TO CYTOLOGY 



of substance, microchemical tests indicate that it cannot be nucleic 

 acid, "chromatin," or "karyotin." Heitz, who reports that in Vicia 

 both nucleolus and secondary constriction react negatively in the Feulgen 

 test for nucleic acid,'^ inclines to the view that the nucleolus is formed 

 by a substance existing throughout the nucleus and later accumulated 

 near the constriction. In Zea sporocytes it is clear that the nucleolus 

 bears a very close relation to the chromosome matrix (Fig. 65a). An 



i i -^ 



AT 



n 



Fig. 65a. — Chromosome matrix and nucleolus in Zea. A-D, four stages in prophase 

 of second meiotic division in microsporocyte. The nucleolus and the matrices of the 

 several chromosomes are at frst confluent. The distinction between matrix and chro- 

 monema shows well in B and C. E, normal microspore with one principal nucleolus. 

 F, microspore lacking the region of chromosome VI specially concerned in nucleolus 

 formation; many scattered nucleolus-like bodies present. {After McClintock, 1934.) 



association of nucleolar behavior with the transfer of products of genie 

 activity to the cytoplasm has been suggested by Zirkle (19286) and 

 Fikry (1930) (see p. 309). 



The observations and hypotheses cited above indicate that certain 

 chromosomes of the complement have regions specialized in connection 

 with the transfer of material to and from them during the nuclear cycle 

 (Haase-Bessell, 19286). Accordingly, it might seem that every nucleus 

 must have at least one such chromosome in order to pass through a 



* Negative tests for the nvicleolus are also reported by Wermel (1927), Yamaha 

 (1932), Shinke and Shigenaga (1933), and others. 



