No. 3.] MORPHOLOGY OF THE ACTINOZOA. 325 



{Nesscldriisenstrcif) which is first formed, and for some time it 

 is the only portion of the filament which is present. I shall 

 consider its formation first, reserving a description of the forma- 

 tion of the lateral streaks {Flimnicrstreifcn) until later. 



In an embryo in which the second cycle of mesenterial pairs 

 is making its appearance, the first four pairs of mesenteries are 

 already provided with fully formed filaments. The fifth and 

 sixth pairs are just forming them, while in the mesenteries of 

 the second cycle, which are yet quite small, there is no trace of 

 them. It is certain that the reflection of stomatodaeal ectoderm 

 described by H. V. Wilson does not exist. The mesenteries of 

 the fifth and sixth pairs have reached the stomatodaeum and 

 have fused with it for about the upper half of its length ; below, 

 however, they are still separate from it. In a section which 

 passes through the stomatodaeum immediately below the point 

 of separation of the mesentery from it (Fig. 21), it can be seen 

 that no ectoderm exists upon its outer surface, while in sections 

 lower down (Fig. 22), the mesenterial filament is seen. There 

 is therefore no connection between the stomatodaeal ectoderm 

 and the cells forming the free edge of the mesentery. The con- 

 clusion is evident that the median streak of the mesenterial fila- 

 ment is a product of the endoderm. 



I have verified this result in the case of the third mesenteries 

 in younger larvae, and have no reason to doubt its accuracy. 

 The statements of H. V. Wilson ('88) and E. B. Wilson ('84) 

 are both so definite, and at the same time so absolutely opposed 

 to one another that it is of importance to determine, by the 

 study of a number of forms, what actually takes place. It does 

 not seem probable that structures having the same morphologi- 

 cal and physiological characteristics should be derived in such 

 nearly related forms as the Alcyonaria and Hexactinians from 

 two different germ-layers, and the question seems to be as to 

 the correctness of the interpretation of the observations made 

 by the authors mentioned. What I have found inclines me to 

 accept the observations of E. B. Wilson. 



The filaments, when first formed, consist, as stated above, 

 solely of the median streak. This is small at first, and differs 

 considerably from the structure of older filaments, as can be 

 seen from a comparison of Figs. 20 and 23. In the younger 

 filament (Fig. 20) the cells are elongated, differing markedly 



