Gametogenesis in Cestodes, 
435 
function of the chromosome in inheritance and return to the simpler con- 
ditions of many unicellular forms. Nor would the failure of the chromosome 
to play its accustomed role in these degenerate forms “deal a fatal blow“ 
to the chromosome theory based upon mitotic processes in higher types. 
I have considered above the possibility that these observations are 
based on artefacts, and dismissed it for what appear to me sufficient reasons. 
A third alternative is that the degenerating skeins play no part in 
normal spermatogenesis, which takes place by mitosis in the customary 
way. This hypothesis is rendered improbable by the frequent occimrence 
of skein degeneration, together with the rarity or absönce of later mitotic 
stages, in all of the large number of proglottids of seVeral species which 
I have studied at the proper stage of development. 
Certain it is that mitosis does occur frequently in certain stages of 
cestode development, and very rarely or never in other stages; while it 
is apparently niore frequent in some forms than in others, which conditions 
agree well with the theory that mitosis is at present degenerating in this 
group. And the observations here recorded are difficult of explanation 
by any other hypothesis than that of mitotic degeneration. 
Conclusions. 
A. Evidence of degeneration of mitosis in cestodes is furnished by 
1. Its irregularity of occurence in 
a) Different proglottids of the same chain, 
b) Different chains of the same species, 
c) Different species. 
2. The frequent if not universal degeneration of the skein in what 
corresponds to the prophase of the first matui'ation division in 
spermatogenesis. 
3. Degeneration or absence of the maturation mitoses in oogenesis, 
and the absence of polar bodies. 
B. Degeneracy of mitosis in gametogenesis in cestodes is a functional 
adaptation, correlated to degeneracy in many other respects, and 
accompanying a great increase in production of sex cells, their 
rapid development, and the elaboration of yolk by the eggs. 
C. Such degeneracy of mitosis in no way affects the chi’omosome 
theory as applied to higher animals, but is a reversion to the 
simpler conditions frequent in Protista. 
D. Positive proof of amitosis is impossible in fixed material. 
E. Spermatozoa of cestodes (and trematodes) are the simplest known 
type of sperni cells in animals. 
