DEGENERATION OF YOLK GLANDS AND CELLS IN CESTODES. 31 1 



render it impossible to say that no yolk is present at this time. 

 It is not evident at this time however nor is its presence to be 

 expected in early stages of the yolk gland, before any consider- 

 able differentiation of its cells has taken place. The nucleus of 

 the adult yolk cell apparently offers further evidence of degener- 

 ation for it is very irregular and broken in form and contains 

 but little chromatin, though how much of this appearance is 

 natural and how much artificial, due to imperfect action of the 

 fixative on exceedingly plastic structures, I cannot say. In 

 earlier stages however the yolk cell nuclei are fairly definite 

 structures in similarly fixed material, so I incline to the belief 

 that their irregular appearance in later stages is due to degen- 

 eration. 



I have pointed out above that there is little constancy in the 

 occurrence of a yolk gland in flat worms. It may exceptionally 

 be present in some form in a group, where otherwise it is absent, 

 and vice versa. The fact that it is almost certainly to be re- 

 garded as a specialized part of the ovary, and that its function 

 may be largely assumed by the latter even when it is present, 

 as in cestodes, reduces its functional importance and conse- 

 quently its selective value, and may therefore increase its vari- 

 ability and render it more liable to degeneration. 



In cestodes the function of the yolk gland has been largely 

 supplanted by the ovary itself, and its absence in one, and ap- 

 parent degeneration in another species may very likely be 

 prophetic of its future degeneration in cestodes as a .group, 

 together with so many other organs. 



A further point of interest in connection with these observa- 

 tions is the fact that in Thysanosoma actinoides, where a yolk- 

 gland is absent, the cell so frequently attached to the ovum and 

 embryo, which has been assumed by several writers to be a 

 polar body, is likewise absent; which is a further argument to be 

 added to those already presented by me 1 for considering this 

 cell a yolk cell rather than a polar body. 



1 Young, R. T., "The Histogenesis of the Reproductive Organs of Tania 

 pisiformis," Zoo/. Jahrb., XXXV., 355-418. 



