Hyatt.] 80 [March 5, 



brane made up of the usual layer of cylindrical and flagellated 

 cells. The larva was round or ball-like at this stage as in the 

 preceding morula. Chal. limbata at the same age had a similar 

 form with collared and flagellated outer lnyer and the centre was 

 filled with cells which were more irregular in size, and surrounded 

 in part by granules. Among these small cells occurred a group 

 of clear cells of larger size with very large nuclei lying in an 

 irregular group close to the inner side of the external layer. 

 They were similar to the cells in MetschnikofFs figure 17a (ibid, 

 pi. 23), and the form and structure of this ovum was similar 

 to the larva (L 16) from which these cells were taken, and like 

 this also it was still in its chorion. This embryo, also with the 

 exception of the cylindrical form of the cells and the abundant 

 protoplasm in the interior was a counterpart of the blastula of the 

 Echinoderms figured by Selenka (Zeitschr. Wissen. Zool., vol. 

 xxxiii, pi. 5, f. 2), and the mesoderm cells are similar, but more 

 irregularly arranged. This stage is evidently developed out of a 

 Parenchymula, and the internal layer must be looked upon as 

 derived from delamination, at least in part from the endoblastic 

 cells. Schultze figures several larvae in this stage, especially a 

 section of the larva of Aplysina sulfurea (Zeitschr. Wissen. Zool., 

 pi. 24, f. 30), but his connective web of granular protoplasm was 

 not visible in our larvae. The larva of Hym. caruncula was 

 older and the internal cells were similar to those of the denser 

 homogeneous contents figured by Metschnikoff in the larva of 

 Halisarca Dujardinii (ibid. pi. 20, f. 12.). In an evidently older 

 and larger larva of Chal. limbata the form had become elongated 

 or oval, though no indications of the collar had yet appeared. 

 The internal contents had assumed the homogeneous cellular 

 aspect of the larva as just described, in Hymeniacidon and 

 Halisarca. 



These facts, and those given farther on with regard to the per- 

 manent gastrula in Calcispongiae, make it probable that the inva- 

 gination occurs after the formation of the ciliated endoblast and 

 possibly at earlier stages in some species. That it may be consid- 

 ered as instrumental in forming the eudoderm and is correla- 

 tive with the collar in the Cintoplcnula is also evident. That the 

 gastrula precedes the collar in development is shown by the facts 

 cited above and also the following. In two of our specimens 



