400 
Spirochaetes of Tapes aureus 
only occur at the onset of a period of longitudinal division, so that 
Borrel’s interpretation cannot be accepted. 
Some stages in the gradual separation of daughter forms of different 
individual Spirochaetes undergoing longitudinal division ai’e shown in 
PI. YI, Figs. 41—44. 
Transverse division is brought about by constriction in the middle 
of long forms. Sometimes the constriction is a gradual one (PI. VI, 
Fig. 52), accompanied by slight lengthening of the parent organism, 
and two daughter forms with pointed ends result (Fig. 17). At 
other times the constriction appears to be an abrupt one (Fig. 51) and 
daughter forms, with one end more or less rounded, result (Fig. 26). 
(Cf. Fantham, I. 1908, PI. 3, Fig. 19, and Schellack, 1909, PI. IV., Figs. 
51, 55.) 
No septum was seen in any case of transverse division of Spiio- 
chaetes examined. 
The doubling of a Spirochaete on itself, as represented in PI. VI, 
Figs. 45—47, is suggestive of division. However, I did not see division 
of the organism occur during my observations on living Spirochaetes 
in this condition (see p. 394). 
Explanations of the Morphological Variation of the Spirochaetes 
of Tapes aureus. 
(a) The Hypothesis of Species Differences. The hypothesis that 
the different forms belong to different species is unsatisfactory, because 
the examination of a very large number of Spirochaetes from many 
specimens of Tapes aureus shows that there are no separate or distinctive 
differences between the various forms. The Spirochaetes can be 
arranged in a continuous series, no matter what particular external 
feature of the parasite be used as a test. Where, then, in such a 
series can the limits for species be applied ? It is obviously artificial 
to create barriers between forms that gradually merge into one another 
in nature. 
(b) The Hypothesis of Sexual Polymorphism. That male, female 
and indifferent forms of Spirochaetes occur in Tapes aureus would 
doubtless be asserted by those who believe that sexual forms of Spiro¬ 
chaetes exist. “ Male ” individuals would be represented by the A 
forms (Text-fig. 2 A, PI. VI, Figs. 1—5), “female" forms would be 
represented by the C forms (Text-fig. 2 C, Figs. 34—37), while 
‘‘ indifferent ’’ forms would be found in group B. The great variation 
