464 



HISTOLOGY 



FIG. 431. Asterias Forbesii. Fully 

 grown ovum (primary oocyte), show- 

 ing large chromatic nucleolus and 

 smaller, dark staining group of chro- 

 mosomes. Lower magnification than 

 preceding figure. X 500. (Drawn 

 by H. E. JORDAN.) 



time. This stage appears very regu- 

 larly in the course of development of 

 all the ova. The chromatin portions 

 begin at this time to decrease in size, 

 and soon they come to lie in one, two, 

 or even three small groups near the 

 periphery of the nucleus as in Figure 

 430, C. They will now be called chro- 

 mosomes, because they are supposed to 

 be the individual chromosomes, small 

 and condensed in form, w r hich afterward 

 take part in the maturation divisions. 

 When seen to the best advantage, they 

 appear as tiny, bi-lobed bodies lying side 

 by side. 



The ovum, shortly after this, attains 

 its full size, as seen in Figure 431, and 

 the chromosomes now appear as a very small mass indeed, and are 

 easily overlooked in the large nucleus, especially in unfavorably stained 

 specimens. They seem to lie in almost any part of the nucleus, al- 

 though they are oftenest 



next to some part of the '~~~^\ ^ N \ ' , ' ' 



nuclear membrane. In w ^' > N ^i', // -' ^ ' 



order to determine their 

 presence and number, 

 one must study a com- 

 plete series of sections of 

 any particular egg nu- 

 cleus. Sometimes they 

 are attached to the nu- 

 cleolus and sometimes 

 are farthest from it. As 

 has been said, they may 

 form one or more groups. 

 The figure shows them in 

 a single group. 



The egg is now ready 

 for the maturation divi- 

 sions. The nucleus has 

 become situated near the 

 periphery of the ovum, 

 and the ovum is ready 

 at any time, upon a mus- 



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FIG. 432. Asterias Forbesii. Portion of an ovum just be- 

 ginning the first reduction division. Chromatin leaving 

 the plastin ground-substance of the nucleolus and being 

 added to the chromosomes near the forming spindle. 

 X 1500. (Drawn by H. E. JORDAN.) 



