120 SILICISPONGIJE. 



The young sponges at a somewhat later stage have been studied 

 by Schulze and Barrois. They are formed of an external layer of 

 flattened cells, not clearly ciliated as in the adult, within which are 

 a normal mesoblastic tissue, and several spherical chambers lined by 

 ciliated cells exactly like the ciliated chambers of the full-grown 

 sponge. Irregular invagiuations of the epiblast give to the young 

 sponge a honeycombed structure. The ciliated chambers in the 

 youngest condition of the sponge are closed ; but in slightly older 

 examples they come into communication with the passages lined by 

 epiblast, and so indirectly with the external medium. 



CeratospongiSB. Amongst the true Ceratospongise the embryos of two 

 of the Aplysinidse, and of Sponejelia and Euspongia have been to some extent 

 worked out by Barrois and Schulze. The form worked out by Barrois is 

 called by him Verongia rosea. The segmentation is nearly regular, but from 

 the first the segments may be divided according to their constitution into 

 two categories. At the close of segmentation the embryo is oval and 

 covered by a single layer of columnar ciliated cells ; these cells may however 

 be divided into two categories, corresponding with those observable during 

 the segmentation. A certain number are coloured red and form a definite 

 circular mass at one pole, while the remainder, which constitute the major 

 part of the embryo, have a pale yellowish colour. Those at the red pole 

 lose their cilia in the free larva, but around the area formed by them is a 

 special ring of long cilia. The chief peculiarity of the embryo (made known 

 by Schulze) consists in the fact that the layer of cells which covers the 

 embryo does not, as in other sponge embryos, simply enclose a space, 

 but the interior of the embryo is formed of a mass of stellate cells like the 

 normal mesoblast of full-grown sponges. 



This feature is also characteristic of the embryos of Spongelia and 

 Euspongia. 



The embryo of the Gummineje (Gummina mimosa) has been in- 

 vestigated by Barrois (no. 122), and has been shewn closely to resemble the 

 typical larvse of calcareous sponges ; one half being formed of elongated 

 ciliated cells and the other of rounded granular ones. 



SilicispongiSB. The development of marine silicious sponges is but 

 very imperfectly understood. The larvse of various forms Reniera (Iso- 

 dyctia), Esperia (Desmacidon), Raspailia, Halichondria, Tethya have been 

 described. Barrois has shewn that the egg segments regularly and that in 

 the earlier stages a segmentation cavity is present. In the later stages the 

 embryo appears to become solid. Externally there is a layer of ciliated cells, 

 and within a mass of granular matter in which the separate cells cannot be 

 made out. The granular matter projects at one pole, and forms a prominence 

 possibly equivalent to the granular cells of Sycandra. In some forms, e.g. 

 Reniera, the edge of the unciliated granular prominence may be surrounded 

 by a row of long cilia. In later stages the granular material may project at 

 both poles or even at other points. One remarkable feature in the 

 development of the Silicispongise is the appearance of spicula between the 

 ciliated cells and the central mass, while the larva is still free. 



Professor Schulze has informed me that these spicula are developed 

 in mesoblast cells ; while the horny fibres of the sponge are developed 

 as cuticular products of special mesoblast cells (spongioblasts). 



