MONOGRAPH OF THE EXISTING CRINOIDS. 391 



of the cell. During its actual disappearance the yolk nucleus is frequently repre- 

 sented by several detached areas, but in all such cases these regions are so placed 

 with reference to each other as to indicate that they are not detached fragments, 

 but persisting regions of the original yolk nucleus, the unequal disappearance of 

 the last traces of the basophile reaction having led to their isolation. Very 

 shortly after this stage the yolk nucleus disappears from view altogether, and 

 no trace of it is ever to be seen on the completion of yolk formation. 



In the above, description of the behavior of the yolk nucleus no mention has 

 been made of a peculiar feature in connection with this structure which is of 

 very frequent occurrence. After most fixing reagents the yolk nucleus is accom- 

 panied, usually on its outer side, by a clear space in the cytoplasm. On the first 

 appearance of the definitive yolk nucleus, while this is still faintly staining, this 

 space is absent, the cytoplasm in the immediate neighborhood of the yolk nucleus 

 in no way differing from the rest. This condition is maintained throughout the 

 subsequent history of the yolk nucleus in material fixed solely in a saturated 

 solution of sublimate, but after most other fixing reagents a clear area appears 

 on the outer side of the yolk nucleus and grows step by step as the latter increases 

 in staining capacity, and only finally disappears shortly before the disappearance 

 of the yolk nucleus itself, slowly decreasing in extent as the latter loses in staining 

 capacity. In the later stages the clear space, like the yolk nucleus itself, is less 

 regular in form, and though usually or mainly on the outer suface of the yolk 

 nucleus, may occasionally completely surround it, or even be confined to its inner 

 side, though this latter condition is rarely seen. 



The gradual growth of the clear space, hand in hand with the increase in 

 basophility of the yolk nucleus, and its disappearance with the decline of this 

 reaction that is, its coincidence with the period of apparent greatest activity 

 of the yolk nucleus seems at first sight to suggest that the clear area stands 

 in some direct physiological relation to the activity of the yolk nucleus, and 

 represents an accumulation of noncoagulable fluid in the living egg. There are, 

 however, several difficulties in the way of such an interpretation. 



In the first place, the space is entirely absent in sublimate fixed material, 

 though present in material taken from the same individual, but fixed with an acid 

 reagent. The absence of the clear space in sublimate material can not be due to 

 the great coagulating power of this reagent having coagulated the fluid accumu- 

 lation, for not only would such a well-marked segregation of material so different 

 in constitution from the rest of the cytoplasm show some difference in staining 

 capacity from the latter, a difference of which there is no trace, but there is 

 frequently in this material no room for such an accumulation on the outside of 

 the yolk nucleus. 



The clear space, therefore, is wholly unrepresented in sublimate material, 

 and its presence in other material must be an artifact due to shrinkage during 

 fixation. Indirectly the absence of the clear space in sublimate material is due 

 to the greater capacity of this reagent for fixing simple proteids in the absence 

 of a free acid; for the coagulation of these waste or metaplastic substances with 

 which the yolk nucleus and general cytoplasm are loaded prevents the shrinkage 



142140 21 Bull. R2 27 



