MONOGRAPH OF THE EXISTING CRINOIDS. 



393 



which produces this space in other material. That this is the explanation of the 

 absence of the clear space in sublimate material is very clearly shown by the fact 

 that even in acid-fixed material there is no clear space accompanying the yolk 

 nucleus when this structure is faintly stained, and when it is, therefore, presumably 

 less heavily loaded with noncoagulable material. 



In the. second place, both the clear space and the yolk nucleus of the older 

 oocytes have lost the regular form they possessed in earlier stages, and it is then 

 frequently seen that the outline of the clear space corresponds exactly with that 

 of the yolk nucleus, a correspondence which can only be explained by supposing 

 that the two surfaces were originally in contact and have been separated by 

 shrinkage and is wholly incompatible with the supposition that the space is caused 

 by an accumulation of fluid material in the living egg. This interpretation of 

 the space as a split due to shrinkage is also supported by the presence, on the 

 surface of the yolk nucleus facing the space, of numerous frayed strands of 

 cytoplasm. The extensive changes in the form and size of the yolk nucleus make 

 it impossible to ascertain by direct measurements, on acid-fixed and sublimate- 

 fixed material from the same individual, to what extent the shrinkage of the yolk 

 nucleus is responsible for the clear space; but the fact that there is no great 

 difference in size between yolk nuclei in oocytes of about the same age, the one 

 accompanied, the other unaccompanied, by a clear space, shows that this con- 

 traction can not be a very extensive one, and an explanation other than the con- 

 dition of the yolk nucleus must be found for the very large clear space often 

 present. The explanation is given by two figures in Professor Chubb 's paper 

 which clearly show that the contraction of the egg cytoplasm as a whole may often 

 be largely responsible for augmenting the size, of the clear area. In one the yolk 

 nucleus is accompanied by a clear space of a size, and in this particular case of 

 a shape, wholly inconsistent with the assumption that it is due entirely, or even 

 mainly, to the contraction of the yolk nucleus. In the other the general con- 

 traction of the cytoplasm has occurred on the side of the egg remote from the 

 yolk nucleus and shows that the clear area as seen in the first is due to the combined 

 contraction of both yolk nucleus and cytoplasm. 



The clear space which often accompanies the yolk nucleus, usually on its 

 outer side, is therefore an artifact and is due to the shrinkage which results from 

 the inability of the acid-fixing reagent to coagulate the waste and metaplastic 

 substances with which the yolk nucleus and general cytoplasm are loaded. When 

 the yolk nucleus is faintly stained, as on its first appearance and shortly before 

 its disappearance, at times, therefore, when it is less heavily loaded with these 

 substances, it is unaccompanied by a clear space in any material. 



It is not necessary to assume for the yolk nucleus an active participation in 

 the cell metabolism in order to explain the changes in form and position which 

 this structure undergoes during yolk formation, for these changes are perfectly in 

 accord with the assumption that they are entirely passive in character and due 

 to the wradual change in the physical consistency of the cytoplasm which precedes 



