CNIDARIA 



91 



development of the pairs of septa first to appear must be 

 modified in accordance with the conjectures of 0, UNr> R. 

 Hertwig (No. 9), which have been confirmed by the observa- 

 tions of H. V. Wilson (No. 99) and others. The sequence in 

 the development of the different pairs of primary septa is 

 consequently as follows. At first a pair of septa arises which 

 is placed nearly at right angles to the elongated oral fi^ssure 

 which marks the plane of symmetry (Fig. 40 i). This pair of 

 septa is formed as a longitudinal fold of the entoderm, inside 

 of which there extends a process of the gelatinous sustenta- 

 tive lamella. By the development of this first pair of septa, 

 which lies nearer to one oral angle than to the other, the 

 peripheral part of the gastral cavity is separated into two 

 gastral pouches, one of which is smaller than the other. By 

 means of the second pair of septa (Fig. 40 2) the larger of the 

 two pouches is separated into three parts. The third pair of 

 septa is developed in the smaller of the two primary gastral 

 pouches, and divides this in like manner into three parts, 

 whereas the fourth pair of septa is developed in the unpaired 

 pouch which is enclosed by the septa No. 2 (Fig. 40 3 and 4). 

 This stage with four pairs of septa marks a kind of resting 

 phase in the development. Up to this time the septa were 

 always established in pairs, and in such a way that each new 

 pair was developed in one and the same gastral chamber. 

 For the pairs which now follow, Nos. 5 and 6, the statements 

 of H. V. Wilson (No. 99) and A. C. Haddon (No. 77) agree 

 with those of Lacaze-Dqthiers to the effect that they take 

 their origin in the two pairs of chambers which lie next to the 

 pair of septa first formed. Accordingly the septa of these 

 two pairs would make their appearance independently in four 

 different gastral pouches (Fig. 40 B). On the other hand, the 

 brothers Hertwig (No. 9) have observed in Adamsia diaphana 

 another mode of development of these two pairs of septa, 

 both of which here arise in the chambers lying between septa 

 1 and 2 (Fig. 41). Accordingly even in the Hexactinise alone 

 dift'erent conditions seem to prevail regarding the arrange- 

 ment of the longitudinal muscles on the first eight septa 

 and the development of the fifth and sixth pairs of septa.^ 

 ^ [The recent investigations of Boveri (No. III., Appendix to Literature 



