THE AMPHIBIA OF OKLAHOMA 



and in slightly smaller oocytes of B. cognatus. No zona radiata, frequently 

 found within the zona pellucida of vertebrate eggs, could be demonstrated 

 as present in these species. After ovulation, the zona pellucida becomes the 

 vitelline membrane. 



In any mature ovary, practically every stage of primary oocyte growth 

 can be found from that of the young oocytes which are attached to the sac 

 to that of the older and larger cells which are packed together in the ovarian 

 cavity. The development of individual cells in the ovaries of B. cognatus and 

 B. w. woodhousii is very similar for the two species; therefore, a general 

 blanket description of these events is sufficient. 



In the oocytes of B. cognatus ranging up to 0.34 mm., the cytoplasm 

 appears entirely homogeneous and without inclusions. The nucleus is round 

 or ovoid in shape, bears inclusions, and is surrounded by a definite nuclear 

 membrane. With the beginning of yolk synthesis, however, the oocytes show 

 decided changes in both cytosome and nucleus and steadily increase in size 

 until a maximum diameter, approximately 1.1 mm., is reached. 



The first alteration in the cytoplasm occurs when small and irregularly 

 shaped vacuoles appear in the form of a ring near the periphery of the cell. 

 This condition is apparently of brief duration in both species. As growth 

 continues, fine yolk granules appear near the zona pellucida, increasing in 

 number and size nearer the nuclear membrane. Eventually the cytoplasm 

 becomes obscured from view by the heavy yolk globules. lust within the 

 oocyte membranes, however, a very narrow layer of cytoplasm remains which 

 is never invaded by the accumulating food material. This pattern of yolk 

 formation is consistent in all large cells and in general excites less attention 

 than do the events which occur concomitantly in the nucleus. 



The original smooth-surfaced nucleus becomes highly irregular in shape 

 as development continues, and the reticulate and lightly staining chromo- 

 somes soon can be located in the central part of the nuclear body. Occasionally 

 one may see as few as three or four nucleoli in the small oocytes; more fre- 

 quently the number is very great. In one oocyte of Bufo cognatus measuring 

 0.085 mm. in diameter whose nuclear diameter was only 0.031 mm., thirty- 

 four nucleolar bodies of various sizes were counted; and this condition seemed 

 not exceptional in either B. cognatus or B. u>. woodhousii. With the begin- 

 ning of oocyte growth, these nucleoli, which originally assumed no particu- 

 lar arrangement, form a ring near the nuclear membrane and then gradually 

 proceed inward toward the centrally located chromosomes, maintaining the 

 ring position during this movement. Midway into the center of the nucleus, 

 the number of nucleoli becomes greatly increased and their size range is 

 vastly extended. Further, their staining reaction begins to vary. Many of the 

 larger nucleoli stain very lightly at this time, meanwhile becoming vacuo- 

 lated; therefore, many large, darkly-staining nucleoli appear to discharge 

 minute dots of nucleolar material both toward and away from the nuclear 

 center. This process continues up to the ovulation period. Then quite sud- 

 denly, the entire mass of nucleolar structures seems to disappear. The nuclei 

 of the oocytes retained in the ovarian cavity after ovulation entirely lack 



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