THE FEMALE 



61 



FROG: 



Similarity of frog and of salamander chromosome structure. Stage 4 chromo- 

 somes. There is apparently a single chromonema along which compound gran- 

 ules and chromomeres of varying shapes and sizes are firmly embedded or at- 

 tached. These chromosomes were treated with 0.2 M NaHCOa to remove the 

 lateral loops and reveal the chromonemata. Chromosome granules are attached 

 to the paired chromonemata at homologous loci. Some matric coating is present. 

 Mild acidification following carbonate treatment has removed most of the lateral 

 loops. (Courtesy, W. R. Duryee, 1950, Ann. N. Y. Acad. ScL, 50, Art. 8.) 



resynthesized on the cytoplasmic side of the membrane. During the 

 growth of the oocyte, further nucleoli appear within the nucleus, 

 only to fragment and later to pass out into the cytoplasm. The 

 presence of chromatic nucleoli in the cytoplasm is closely associated 

 with the accumulation (deposition) of yolk. 



The granules within the cytoplasm (extruded fragments of nucleoli) 

 function as centers of yolk accumulation and have therefore been 

 named "yolk nuclei." This is an unfortunate name, for the structure 

 is a nucleus only in the sense that it is a center of aggregation. It is 

 not a true cell nucleus. The centrosome and other granular centers 

 lose their identity and the yolk granules then become scattered 

 throughout the cytoplasm. 



The source of all yolk for the growing ova is originally the digested 

 food of the female. This nutrition is carried to the ovary by way of 

 the blood system and conveyed to the nurse or follicle cells and thence 

 to the oocyte. The yolk is at first aggregated around yolk nuclei, then 

 concentrated to one side of the nucleus. Finally it assumes a ring 

 shape around the nucleus between an inner and an outer zone of 

 cytoplasm. Subsequently the nucleus is pushed to one side by the 

 ever-increasing mass of yolk so that eventually there is an axial 



