Brown — Developmental Stages in Streptelasma rectum. 279 



can be affirmed that in all cases as soon as any of the primary 

 septa are determinable they are already six in number, all fully 

 developed, practically equal in size, and radially disposed at 

 equal distances apart. Two pairs never appear in advance of 

 a third pair. Moreover, there is never any hint of the third 

 pair being inclined at its origin towards the others, after the 

 manner of development invariably characteristic of the first 

 and later pairs of the metasepta."* 



The foregoing statements of observations indicate that Dner- 

 den is wrong in these contentions. In the first place an earlier 

 four septal stage can be found : secondly, the third pair of 

 septa do appear later and are therefore the first pair of second- 



ary septa (metasepta) : thirdly, this first pair of secondary 

 septa are inclined towards the alar septa on their dorsal side 

 and develop in the same manner as any other pair of meta- 

 septa. 



Fig. 3 shows the appearance of a second secondary septum 

 in one counter quadrant. Fig. 4 shows two secondary septa 

 in each counter quadrant. Fig. 4a is the same stage from 

 another individual and shows that a pair of tertiary septa 

 (exosepta) have already appeared, one on either side of the 

 counter (dorsal directive) septum. Attention is especially 

 called to this very early appearance of the first pair of ter- 

 tiary septa adjacent to the counter septum. Fig. 5 shows the 

 appearance of the third secondary septum in one counter quad- 

 rant and the appearance of a tertiary septum in the same 

 quadrant. In fig. 6 we see a tertiary septum present on 

 either side of the counter septum, three secondary septa in 

 either counter quadrant, and one secondary septum in each 



* Annals and Magazine of Natural History, series 7, vol. xviii, p. 236, Sept. 

 1906. The Morphology of the Madreporaria. The Primary Septa of the 

 Eugosa, J. E. Duerden. 



