MAMMALIAN CESTODES. 567 



appearance, and seems to lie in concentric layers surrounding the 

 embryo ; in others this is not apparent, and it even simulates a 

 retiform tissue in the Lixity of its arrangement. 



The ultimate aiTangement of the uterine spaces in this worm 

 is, therefore, the same as we find in the species of Dipylidium 

 that has just been described. But the two genera diifer in the 

 fact that this is preceded in Dijyylidhtm by another developmental 

 stage not repi^esented in Diplopylidium. In the latter we find 

 the scattered cavities lying among the remains of the discharged 

 testes, and as both a final and an initial stage. There is no inter- 

 mediate condition that I have found where the ova are contained 

 in larger cavities, which subsequently become reduced to chambers 

 each containing a single egg, such as is found in Dijyylidium. 

 The cavities in the two cases are, therefore, not exactly equiva- 

 lent ; otherwise, as has been said, there is a close agreement 

 between the two worms. In neither can any lining membrane 

 be detected forming a wall to the egg-cavities. The actual size 

 of the individual spaces is much the same, but in Di.pylidumi 

 they are more closely packed together. In formulating this 

 difference between the two genera I rely upon what I have seen 

 and not seen in my sections. It would be unwise — because of 

 the negativity of my conclusions — to assert this fact in a more 

 positive way by using it as part of the generic definition of 

 Diplopylidium. It is not only in Dipylidium that we find a 

 series of egg-holding spaces like those of Diplojjylidium. The 

 same kind of arrangement occurs in Oochoristica, so named 

 on account of the scattering of the mature ova through the 

 parenchyma. But in this genus there is an obvious uterus which 

 exists before the final scattering of the eggs. Moreover, when 

 imbedded in the parenchyma, the appeaiance is as if the eggs 

 were firmly imbedded in the homogeneous ground material of the 

 medullary tissue and not received into cavities. The eggs, in 

 fact, are immediately surrounded by solid tissue and not by the 

 rest of a cavity. More like the disposition met with in Dipy 

 lidittm and Diplopylidium is that which is to be seen in Mono- 

 pylidium, a genus to which the present genus, as I point out 

 later, is probably related. But it should be remarked, first 

 of all, that the absence or presence of a cavity round the singly 

 imbedded ova is not a matter of great importance. For, as will 

 be seen by an inspection of text-fig. 93, it happens occasionally 

 in DiplojjylidAum that imbedded ova have no cavity round them. 

 This may well be a preliminary state of affairs, the subsequently 

 appearing cavities being due to the growth of the egg into the 

 embrj'o. In this case there will be no doubt whatever that the 

 egg-holding cavities of Diplopylidium axe totally different from 

 those of Dipylidium dongolense, since the latter are formed as 

 subdivisions of an earlier larger cavity. 



In 3Iono23ylidium the ova are in the same way scattered 

 through the parenchyma in cavities which contain one, or in 

 some cases more than one, ovum. It is evident, however, from 



