voL.a.i Robertson. — Embryonic Fission in Crisia. 141 



embryo. In G. eburnea, neither in the primary embryo nor in 

 the buds when first set free, is there any differentiation into cell 

 layers. As the primary embryo increases in size, the eells upon 

 the surface become more compactly arranged, the inner cells 

 forming a loose, spongy mass. The secondary embryos of Crisia 

 denticulata, according to Calvet, possess two distinct Layers, an 

 outer containing large nuclei, and an inner containing much 

 smaller nuclei surrounding a central cavity. This is true even 

 before the buds are detached from the parent. This central 

 cavity persists and forms part, at least, of the genera] cavity of 

 the first individual of the new colony. When the secondary 

 embryos of Crisia eburnea are first set free they do not differ 

 histologically from the primary embryo. No cavity is present, 

 the cells being heaped together in a somewhat irregular way. 

 When a cavity appears it is not at first lined by a distinct 

 layer of cells as is the case in C. denticulata. By the time 

 the ovicell has completed its growth it is filled with larva' of 

 various sizes and in various stages of advancement. Fig. 28 

 is a section through an ovicell which is almost mature, i.e., one 

 in which the Larvae outnumber the embryos and will soon be set 

 free. In this instance many of the larva 1 have attained their full 

 development and are confined in their narrow quarters only until 

 the valvular membrane can be perforated. The larger larva- 

 possess long cilia, which fact suggests that either they move bodily 

 through the ovicell, or that the vibrations of their cilia set up 

 currents which carry the smaller bodies about. It is not uncom- 

 mon to find the secondary embryos remote the length of the 

 ovieell from the primary embryo, showing that the contents of 

 the ovicell must have been in motion during life. The size of 

 the larva? seems to be pretty constant, at least in a given species. 

 Those of G. eburnea measure about 86 ^ in diameter, while those 

 of ('. occidentalis are somewhat larger, measuring 107 p. The 

 opacity of the living ovicell prevents any study of the living con- 

 tents while the ovicell is intact. But if a living ovicell be 

 crushed in a drop of sea water, a very interesting scene 

 is presented. The larvae dart away and swim about with 

 great activity. Smaller ciliated balls move about in clusters. 

 The color of the whole mass, Larvae, embryos, and cellular 



