96 EMBRYOLOGY 



pair of septa is formed in this way, the development of the filaments 

 takes place by simple downgrowth of the ectoderm of the oesophagus, in 

 the direction of the two primary septa. The two gastral pouches first to 

 appear are now completely hollowed out. The new pairs of septa next 

 arise as foldings of the entodermal lamella of the body-wall, and their 

 upper ends seem to be at some distance from the ectoderm of the oeso- 

 phagus, so that no direct outgrowth of the latter can lead to the formation 

 of the filaments. In order to establish the connection between the ecto- 

 derm of the oesophagus and the newly formed septa, the former must bend 

 around at the inner opening of the oesophagus, and grow upward on the 

 outer surface of the oesophagus, until it reaches the uppermost part of the 

 newly formed septa, on to which it now advances to form the filament. 

 This bent-over part of the ectoderm is seen in Fig. 40 A, rf. H. V. 

 Wilson conjectures that the mesenterial filaments of all subsequently 

 appearing septa are formed after this type. 



In general the development of the mesenterial filaments takes place in 

 the same sequence as that of the septa, so that the oldest pair of sejata 

 bears the most develo23ed filaments. 



The tentacles arise as simple evaginations of the body-wall 

 over the different gastral pouches. The sequence of their 

 origin lias been described by Lacaze-Ddthiers (No. 89), 

 especially for Actinia mesembryanthenium. For the early 

 stages it is closely connected with the sequence of the appear- 

 ance of the different mesenteries and the formation of the 

 chambers dependent on it. In this connection ought speci- 

 ally to be mentioned the fact that the tentacle which arises 

 over the larger of the two first-formed gastral pouches con- 

 siderably outstrips the others in development, so that for a 

 long time the bilateral symmetry of the larva is marked 

 externally by the presence of this one large tentacle (Fig. 

 44/1). 



Haacke (No. 76) has called attention to the fact that in attached stock- 

 building forms, as in the blossoms of many Phanerogams, the bilaterally 

 symmetrical fundamental form may be expressed by the position of the 

 buds in relation to the parent animal, i.e., to the axis of the entire 

 colony, since the parts of the bud near to the axis undergo a different 

 development from those remote from it. Moseley had already shown 

 that in Saccophyton and Heliopora the polyps always have their dorsal 

 sides turned towards the axis. We may conclude from such observations 

 that the bilaterally symmetrical structure of the Anthozoa is caused by 

 the formation of stocks. The solitary forms (Actinians) would then have 

 to be derived from those forming colonies. Finally, we may assume that 



