MONOGRAPH OF THE EXISTING CRINOIDS. 



403 



With the commencement of yolk formation, however, this correlation in growth 

 ceases and the cell body grows or, to speak more correctly, expands at a much 

 faster rate than do the other cell structures, so that on the completion of yolk 

 formation the nucleolus measures about 17 [A, the nucleus 50 [x, while the cell body 

 measures fully 160 JA. The following figures represent typical measurements during 

 the progress of yolk formation; they were obtained in the same way as those just 

 given for earlier stages, the mean being taken of a large number of closely ap- 

 proximating measurements for each stage. . It should be noted, however, in con- 

 sidering these figures, that although the nucleolus continues to increase in size 

 perfectly steadily during yolk formation, a corresponding behavior could not satis- 

 factorily be determined for the nucleus, the irregular form of the latter at this 

 period quite precluding accurate measurement. The figures for the nucleus, there- 

 fore, are only approximately correct. 



In Heliometra glacialis, though the eggs are larger, exactly the same relative 

 proportions for the nucleolus, nucleus, and cell body are found. 



Professor Chubb states that one of the most striking features of the young 

 oocyte is the extreme regularity of its spherical nucleus. With no fixing reagent 

 did he ever find this structure in early stages to show the slightest tendency to 

 assume an irregular form. In striking contrast to this behavior of the nucleus in 

 the younger oocytes, this structure in later stages is almost always more or less ir- 

 regular in form, and in some material is markedly so. This irregularity coincides 

 with the period during which the cytoplasm is deeply basophile, becoming more 

 and more pronounced the deeper the cytoplasm stains, and gradually passing away 

 as this staining capacity of the cytoplasm is lost during yolk formation, the nucleus 

 never, however, regaining the regular spherical form characteristic of the earlier 

 stages. The irregularity of the nucleus thus coincides not with the actual process of 

 yolk formation but with the basophile staining reaction of the cytoplasm. 



There are several facts which show that for Antedon this irregularity is a 

 result of the action of the fixing reagent. In the first place, the nuclear irregu- 

 larity shows no spatial relation whatever either to the other cell structures, to com- 

 mencing yolk formation, or to the position of the nucleus in the cell. In the second 

 place, it is only in radial section that the nuclear irregularity presents the appear- 

 ance of pseudopodia; in tangential sections these nuclear processes are found in- 

 variably to resolve themselves into a coarse wrinkling of the nuclear membrane. 

 Finally, the artificial nature of the nuclear irregularity is strongly indicated by the 

 variable behavior of the nucleus with varying fixation. Only in material fixed with 

 sublimate is the irregularity of the nucleus invariably well marked. With other 



