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presenting all the characters of that which we have described 
above (fig. 9); others have, on the contrary, a mobile arm, 
and are devoid of the inert arm (figs. 10, 11, and 12). It 
is to be observed that in this latter case the body of the cytod 
has smaller dimensions than belong to those cytods with two 
prolongations. Among the cytods with only one prolongation, 
some are found with the arm presenting characters inter- 
mediate between those of the mobile and those of the inert 
arm. It results clearly enough, from the comparative exami- 
nation of these various forms, that the inert arm of the two- 
armed cytods is destined to become in turn a mobile arm; 
after the original mobile arm has become detached from 
the cytod. The inert arm is then merely a still younger 
pseudopod than the mobile arm, destined to take on at a 
certain epoch the characters of the latter. 
The fact which is regularly observed in the two-armed 
cytods, that the mobile prolongation thins away progressively 
at its basilar portion (figs. 7 and 8) when it has attained 
a certain length, proves that the prolongation tends to detach 
itself from the cytod; and this conclusion is confirmed by 
the existence of cytods, having only an inert prolongation. 
But this now requires for its development all the rest of 
the body of the cytod. That is at least the conclusion which 
appears to be deducible from the occurrence of free filaments, 
having a vesicular enlargement at their posterior extremity ; 
although no narrowing is observed between the body of the 
filament and the terminal enlargement. 
These facts lead to the following conclusions: 
Ist. Each cytod gives rise to two filaments, destined to 
develop each into a Gregarine; but the development of the 
two processes takes place successively. 
2nd. The filament which develops first attains its ma- 
turity, and detaches itself from the body of the cytod, before 
the other proceeds with its development, and before it attains 
the phase of “ the mobile arm.” 
3rd. This latter does not detach itself from the cytod; it 
develops by gradually absorbing the body of the cytod, as 
the embryo of a vertebrate absorbs little by little the contents 
of the vitelline vesicle. It passes successively through the 
same phases of development as the mobile filament. 
The protoplasmic filaments thus developed from the cytod 
move in the intestine with extreme activity (figs. 13 and 16). 
The only movements which they execute are undulato 
movements, in every respect comparable to those of the Ne- 
matoid worms. In consequence of their resemblance to the 
Nematoid worms, I have termed these protoplasmic filaments 
VOL. XI,.—NEW SER. R 
