H. B. Fantiiam 
399 
rounded, while yet others had their two ends differing from one another 
in their degree of roundness. In nearly every case the membrane and 
its edge could be clearly seen. On examining large numbers of Spiro- 
chaetes from many different Tapes aureus, however, intermediate forms 
exhibiting all gradations of length, breadth and degree of rounding of 
ends can be found (PI. VI, Figs. 1—37). Several hundred Tapes 
aureus were examined, but without finding any substantially new 
variations of the Spirochaetes. 
Division. 
Both longitudinal and transverse division take place, as seen both 
in fresh and stained preparations. Probably there is a periodicity in the 
direction of division followed by the Spirochaetes of Lamellibranchs, 
just as there is a periodicity in the case of the blood-inhabiting Spiro¬ 
chaetes. Miss Porter and I (1909) have found that the Spirochaetes of 
relapsing fevers, S. recurrentis and S. duttoni, divide longitudinally at 
the beginning and end of infection when relatively few Spirochaetes 
are present in the blood, while they divide transversely during the 
height of infection when Spirochaetes are numerous in the blood. 
I have already recorded (vi. 1907, p. 499; I. 1908, pp. 42—46), that 
S. balbianii and S. anodontae divide both longitudinally and transversely. 
The technical difficulties in the way of examining portions of an infected 
Lamellibranch crystalline style from time to time are much greater 
than in examining drops of blood from a small mammal infected with 
Spirochaetes. For this reason it will be difficult to determine periodicity 
in the direction of division of the Spirochaetes of Lamellibranchs. 
4 Longitudinal division of the Spirochaetes of Tapes aureus begins 
by the splitting (PI. VI, Fig. 39) of the membrane lengthwise into 
two (Fig. 40). In such forms the two edges of the membrane are 
clearly distinguishable (Fig. 40). I have found several such Spiro¬ 
chaetes, with double membranes, in the same preparation, and 
not scattered at random through the various preparations made at 
different times. The probability of the occurrence of a periodicity 
in the direction of division of Spirochaetes in the Lamellibranchs 
is thus increased. Borrel (1908) has suggested that the presence of a 
double membrane in Spirochaetes stands in relation to their power of 
reversing their movements. However, all Spirochaetes do not possess 
double membranes; on the contrary, such forms are relatively few, and 
