Vol. XXXVI. May, 1919. No. 5. 



BIOLOGICAL BULLETIN 



THE DEGENERATION OF YOLK GLANDS AND 



CELLS IN CESTODES. 



R. T. YOUNG. 



Yolk glands, while frequent, are not of universal occurrence 

 in flat worms, nor do they apparently bear any constant relation 

 to the systematic position of the various groups of this phylum. 

 In the Accela, which are probably to be regarded as the most 

 primitive of the latter, they are lacking, with the exception of 

 Polychcerus, while the same is true of the polyclads, which are 

 one of the more specialized of these groups. 1 



Among trematodes they are universally present with the 

 exception of Gyrodactyhis, where their absence is possibly corre- 

 lated to the peculiar mode of reproduction, while in cestodes 

 they are typically present, though reduced in some and in one 

 case at least absent. 



Braun 2 in his description of the yolk glands in this class has 

 pointed out their gradual reduction in size from those such as 

 Tetrarhynchus tetrabothrius , in which the gland forms a mantel 

 around the entire proglottid, to those such as many Taeniae, 

 where the gland is reduced to a small unpaired organ adjacent 

 to the ovary. It is this reduction of yolk glands in some cestodes, 

 together with certain corollaries deducible therefrom, that I 

 wish to discuss briefly in this note. 



In a study of gametogenesis in cestodes upon which I am 



1 This last statement holds good even though the polyclads be considered as 

 directly derived from the ancestral form. If the Accela however be regarded as 

 degenerate, rather than primitive types, then the absence of a yolk gland here may 

 be attributed to such degeneracy. This would not explain their presence in Poly- 

 chcerus however, nor their absence in Gyrodactylus, which is exceptional among 

 trematodes in this respect. 



2 Braun, M., "Cestodes" in Braun's Kl. and Ord. IV., Ib. 



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