SPIROSTOMUM AMBIGUUM 401 



A b n o r ni a 1 i t i e s in t li e Form of the M e g a n ii c 1 e u s . 

 — A number of individuals from different cultures have been 

 found in which the meganucleus was abnormal. These observa- 

 tions include individuals of both the major and minor varieties 

 (see below). 



One member of the major variety was found whose mega- 

 nucleus had a short branch, consisting of two lobes and a 

 commissure, given off from one of the commissures. This was 

 the only case of a branched meganucleus met with. 



A fairly common abnormality was the division of the mega- 

 nucleus into two pieces ; in one case three pieces of mega- 

 nucleus were present. Such conditions might be brought 

 about by the snapping of a delicate commissure. Another 

 method was revealed, however, whilst watching a normal 

 individual divide. During the contraction of the meganucleus 

 towards the anterior end of the animal, the posterior end of 

 the meganucleus was seen to come apart from the rest and to 

 follow as a separate small oval fragment in the wake of the 

 rest. Elongation subsequently took place, and, when the 

 constriction of the cell occurred, the anterior daughter con- 

 tained a whole daughter meganucleus, whilst the posterior 

 daughter contained its share of the major fragment and the 

 separated minor fragment. Each of these latter gave rise to 

 a piece of moniliform meganucleus. 



The most interesting cases of abnormalities, however, were 

 found in a five months' old culture of the minor variety of 

 S p i r o s t o m u m a m b i g u u m . The cilia of all were normal, 

 but the protoplasm seemed denser and more granular than 

 usual, although it showed no signs of the vacuolation usually 

 associated with degenerate forms. The meganucleus had lost 

 its moniliform appearance and lay collected in masses in the 

 endoplasm. In some cases it took the form of three or four 

 rounded masses separated from one another by quite distinct 

 gaps. In one case the meganucleus was represented by a big 

 sphere in the anterior end separated by a wide space from the 

 remainder, which took the form of four lobes of the normal 

 moniliform type. 



