1884.] 89 [Hyatt. 



complete in this stage, the embryo may assume the aspect of the 

 next stage if viewed externally. Then (6) a parenchymula in 

 which the cells of the endoblastic hemisphere also become cylin- 

 drical, and acquire collars and flagella ; then (7) a stage of the 

 parenchymula in which a gastrula may be formed by invagination 

 erf the endoblast, and it is probable that the collared and ciliated 

 cells which have been differentiated upon the exterior in the 

 endoblast are carried into the interior to form the endoderm. 

 The blastopore becomes filled with cells, probably part of the 

 invaginated cells of the endoblast; then (8) a cinctoplanula in 

 which a sheaf of spicules or threads is formed in this blastoporic 

 plug, the embryo resumes its oval form, and the cells of the ecto- 

 blast immediately around the blastopore become highly colored 

 and are transformed into a collar; then (9) a stage of the cincto- 

 planula in which the collar is completed, the spicules become 

 more generally dispersed throughout the mesenchyme, the long 

 setae of the collar are formed, and the embryo breaking through 

 the chorion becomes a free larva ; then (10) the ascula, when 

 the larva becomes attached, the ectoblast cells lose their cilia and 

 form the the ectoderm. The primitive cloaca and a central cav- 

 ity, surrounded by an endoderm, appears in the interior; then (11) 

 the ampullinula, in which the central cavity branches and lateral 

 ampullae with collared and flagellated cells are formed by invag- 

 ination together with their incurrent pores. 



The embryo of the Calcispongiae, as described by Barrois and 

 Schultze among Sycones, has at first two cells, then four, then 

 eight. At this stage the mode of segmentation changes ; the 

 cells which have been previously formed exclusively by vertical 

 fission, and are assembled in what we have called a monoplacula, 

 begin to multiply by transverse fission; the upper points of the 

 eight original cells are cut off from the rest and form the apical 

 cells of Schultze, while the lower cells become the basals of the 

 same author. In this way the primitive differentiation of the 

 placula into two layers is established in what we have designated 

 the diploplacula, on account of the flattened form of the embryo. 

 The apical cells are the equivalents of the azygos cell of the Car- 

 neospongiae. The eight apicals spread out at first and form with 

 the basals, which are the equivalents of our coronals, an amphi- 

 morula having apertures above and below. The aula (as we 



