3o8 McMURRICH. [Vol. IV. 



though it seems more probable that they are actually present, 

 though undiscoverable. H. V. Wilson i^Z^), it is to be noticed, 

 found the same difficulty in observing the nuclei of the delami- 

 nated endodermal cells of Manicina. 



As already remarked, the cavity of the blastula becomes more 

 or less filled by a mass of yolk-spherules. Occasionally the 

 appearance of these spherules may occur quite early, but at 

 other times, and perhaps more normally, it is delayed until the 

 beginning of delamination. The granules come from the break- 

 ing down more or less completely of the endoderm cells. That 

 they have this origin is, I think, clearly shown in Figs. 1 1 and 

 12. In the former the disintegration seems to have affected 

 some cells to a very great extent, but in the latter it seems for 

 the most part to be only the central ends of certain cells that 

 are affected. The central mass is certainly not cellular, the cell 

 outline having become obscured, as in certain other Actinozoa, 

 but is composed simply and solely of yolk-granules, sometimes 

 imbedded in a homogeneous coagulable matrix. In some cases 

 where the central matter made its appearance during the blas- 

 tula stage, the inner ends of certain cells seemed to fall off into 

 the central cavity, and then to undergo disintegration, but in 

 no case could there be said to be actual cellular elements in the 

 central mass. This is a point of considerable importance, hav- 

 ing very decided bearing upon what has been described for other 

 Actinians, and allowing of inferences as to the actual occurrence 

 in these superficially studied cases. 



At the conclusion of the delamination a slight depression is 

 to be observed at the posterior pole of the larva (Fig. 9), and 

 soon after this breaks through, a communication of the interior 

 cavity with the exterior being thus established. The endo- 

 dermal cells have by this time arranged themselves in a definite 

 layer, separated by a slight space from the ectoderm, and the 

 larva has the appearance of a typical invaginate gastrula (Fig. 

 10). In fact, before making my sections, I was persuaded that 

 the formation of the germ-layers was by invagination. It is not 

 easy to observe in optical sections the endoderm cells soon after 

 their delamination, and larvas with the slight depression at the 

 posterior end appear to be single-layered (Fig. 9), the depression 

 seeming to be the commencement of an invagination. The fact 

 that I was not able to observe any intermediate stages between 



