278 Brown — Developmental Stages in Str&plelasma rectum. 



figures. All these figures except fig. 1 and fig. 4a are the suc- 

 cessive stages in the development of a single individual. 



Fig. 1 shows the tip of an individual with only the four 

 primary septa present. These septa, however, are not disposed 

 at right angles. The alar septa are inclined toward the cardi- 

 nal, thus leaving the counter quadrant spaces considerably 

 larger than the cardinal quadrant spaces. 



Fig. 2 shows the slightly fractured tip, the first view of the 

 individual that was followed throughout its stages of develop- 

 ment. Here the four primary septa are present and two 

 secondary septa have appeared, one in either counter quadrant. 

 These are distinctly not equal to the primary septa and are not 

 radially placed, hut are short and are joined by their inner 



border to the dorsal side of the alar septa. As they develop, 

 this point of attachment moves inward until they do, in some 

 individuals, become equal in size with the primary septa and 

 are radially arranged. There are in the Columbia collections 

 a series of individuals that show the gradations from the con- 

 dition shown in fig. 2 to six equal and radially disposed septa 

 as shown by Duerden in his earliest stage. 



Duerden in his latest paper states that : " In the above and 

 other species {Lophophyllum p>roliferum, Streptelasma rectum, 

 Cyathaxonia cynodon), in which the septal constitution has 

 been established by the process of grinding, it may be objected 

 that if earlier stages than those first represented could be 

 obtained, four primary septa might then be disclosed, and the 

 other two would be seen to be but later additions to a tetra- 

 meral group ; in other words, that the earliest septal stage is 

 not that represented as such. Were this the case, the dorso- 

 lateral pair here regarded as protosepta would be really the 

 first pair of metasepta. Against this reasonable objection it 



