Pseudogastrula Stage in Calcareoits Sponges. 97 



of the chamber becomes tightly stretched. These relations 

 are of great importance in considering the development of 

 the embryo, and they appear to be perfectly constant. 

 Judging from the figures of Schulze, Barrois, and Metschni- 

 koff, the embryo of Sycandra appears to be very similarly 

 situated. 



The youngest embryos in Grantia lahyrinthica are 

 always found near the margin of the sponge-cup, not far from 

 the last formed flagellated chamber. Figure 1 represents 

 the earliest stage found. The embryo here represented may 

 be considered as in a late blastosphere stage, closely 

 resembling the similar stage in Sycandra raplianus. The 

 layer of elongated columnar cells is strongly arched, so 

 as to form almost a hemisphere and lift up the wall of the 

 flagellated chamber in tlie form of a blister as above described 

 (owing to the direction in which the section happened to be 

 taken this is not very well shown in the figure). The 

 ovoid granular cells still form a single layer, or very nearly 

 so, but show signs of proliferation already. This laj^er 

 is flattened, and it is easy to see from the figure that the 

 flattening is caused by the presence of the layer of rigid 

 spicules beneath them. In this and in all the numerous 

 other embryos which I have examined in the maternal 

 tissues the ovoid granular cells are turned towards the layer 

 of spicules, and the columnar cells towards the flagellated 

 chamber. According to Schulze,* prior to this stage in 

 Sycandra raphanus the layer of granular cells is always 

 turned towards the flagellated chamber, but after this stage 

 he finds the embryo in very different positions in the cap- 

 sule, "gewohnlich sogar mit dem hellen convexen Zellenlager 

 dem Kadialtubus zugewandt," Judging from this change 

 in position, Schulze considers that from now onwards the 

 columnar cells are ciliated. In my sections, made from spirit 

 material, I have naturally enough not observed any cilia. 

 . Even in this early stage of development the segmentation 

 cavity is no longer quite emptj^, but contains a quantity of 

 very delicate, finely granular, gelatinous-looking tissue 

 (Fig. 1 rnes.), in which a number of small, deeply staining 

 nuclei are very distinctly visibk. This tissue appears to be 

 of constant occurrence, and is probably the commencement 

 of the mesoblast, or mesoderm of the adult. It is quite 

 uncertain from which layer it is derived, but the similarity 

 of the nuclei to those of the columnar cells, and the fact that 



* " Zeitschrift fiir wissensch. Zoologie," Vol. xxv. (Supplement), pp. 271, 272. 



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