324 McMURRICH. [Vol. IV. 



mesenteries which have arisen simultaneously, or nearly so, in 

 corresponding regions of each antimere of the bilaterally sym- 

 metrical embryo, and have their longitudinal muscles upon the 

 faces turned away from each other. The other four pairs have, 

 however, a very different constitution. Each consists of one 

 mesentery formed previously to the directives, and one formed 

 after their completion. Whereas in the case of the directives 

 the mesenteries forming each pair are homochromous in their 

 formation ; in the lateral pairs, the mesenteries of each pair 

 are heterochromous. These heterochromous pairs differ from 

 the directives in the position of the longitudinal muscles which 

 are on the adjacent faces of each pair. 



In succeeding stages as Lacaze-Duthiers pointed out ('72) 

 the appearance of new mesenteries is governed by a law different 

 from that which operated up to the conclusion of the Halcampa 

 stage. Up to this period the mesenteries made their appear- 

 ance bilaterally, one on each side of the dorso-ventral line, the 

 tendency towards bilateral symmetry being the governing force 

 which determined the manner of their development. In the 

 stages which now supervene, the new pairs of mesenteries make 

 their appearance as if governed by a radial symmetry, develop- 

 ing simultaneously in the intervals between each of the six 

 Halcampa pairs, the mesenteries of each pair forming contigu- 

 ously. In this way a stage with twelve pairs of mesenteries is 

 formed, having all the characteristics of a Hexactinian. Em- 

 bryos showing the formation of this second cycle of mesenteries 

 are the oldest that I possess. 



As regards the formation of the mesenterial filamentals my 

 observations on Aulactinia are somewhat more definite than 

 those on RJwdactis, though at the same time I do not regard 

 them as perfectly conclusive. Taken, however, into consider- 

 ation with what I have seen in adult Actinians, and with the 

 very definite observations of E. B. Wilson on the development 

 of the filaments in the Alcyonaria ('83 and '84), I am inclined 

 to believe that the median streak of the mesenterial filaments 

 of Aulactinia are developed from the endoderm. 



As already stated, in the youngest Aulactinia embryo mesen- 

 terial filaments are present only on the mesenteries of the first 

 pair, so that I have ample material illustrating their formation 

 on the other mesenteries. In all cases it is the median streak 



