C. M. CHILD. 



this kind. In a study of cytological literature it is difficult to 

 resist the impression that some of the apparent uniformity in 

 cytological phenomena which appears so remarkable and mys- 

 terious is in reality the result of the selection of certain " typical ' 

 cases and the discarding of others. While I should not venture 

 to assert positively that the number of chromosomes in either of 

 these species of Moniezia is actually variable, this certainly appears 

 to be the case, and the evidence given in support of the conclusion 

 that the number of chromosomes in a given species or variety is 

 invariable has not always carried conviction to my mind. 



However, the description of the spermatocytic divisons is not 

 the chief object of the present paper and although I have spent 

 much time in endeavoring to reach' well-founded conclusions in 

 regard to the number of chromosomes in Moniczia it has not 

 been possible for M. pianissimo.. In M. expansa, as noted above, 

 the maximal number counted during maturation was eight. 



In some of the nuclei at this stage these chromosomes appear, 

 as in many other forms in a more or less regular grouping about 

 the periphery of the nucleus. The figures do not show this con- 

 dition particularly well but Figs. 14, A, 14, B, 16, A, 16, C (PI. 

 XL), were drawn from nuclei in which the chromosomes were 

 thus arranged. Such nuclei are recognizable at once in a section. 

 This condition, however, was not very commonly found ; much 

 more frequent was the condition shown in Fig. I 5 where they were 

 irregularly disposed, some at the periphery, others near the cen- 

 ter. It is possible that the stage of peripheral arrangement is of 

 relatively short duration and is therefore less frequently seen, but 

 there is also a possibility that it is not of universal occurrence. 

 The question may be left open for the present. 



In Fig. 17 (PI. XI.) two of these chromosomes are shown, both 

 from nuclei with membranes still intact. At the stage figured 

 the fusion of the two parts is not yet complete. So far as actual 

 observation goes these chromosomes of the first maturation divi- 

 sion are dyads, not tetrads, no indication of quadrivalence hav- 

 ing been observed at any stage, and although they correspond as 

 regards ultimate fate with the tetrads of various authors, I prefer 

 to designate them as what they appear to be --dyads. In con- 

 sequence of their small size, almost spherical form in late stages, 



