324 



NATURAL HISTORY. 



briskly about by the movements of its flagella. It then settles clown on some solid body, and 

 flattens out into a disc, which becomes attached by pseudopodia-like extensions from the flagellated 

 cells around its margin. The flagellated cells, losing their flagella, form the pavement-like cells of the 

 ectoderm. Soon after attachment, the hypoblast splits in the middle, and the cleavage cavity so 

 produced becomes lined by flagellated cells differentiated from the hypoblast (Fig. 12, c, D). This 

 layer of cells is the primitive endoderm, the remainder of the hypoblast is the mesoderm. The 

 endoderm buds off flagellated chambers into the mesoderm, and becomes itself converted into plate 



Fig. 13. DEVELOPMENT OF A CALCAREOUS SPONGE (Sljcandm raphanus). 



A, B, Ovum segmented into eight cells : C, side view of embryo in the blastula stage, eight of the granular cells which give rise to the ectoderm 

 of the adult are present at the lower jiole ; D, amphililastula stage : K, a later glace, after the ciliated cells have commenced to become 

 invaginated; F, fixed gastrula stage. The figure shows the amoeboid ectoderm cells (ec'i derived from the granular cells of the earlier 

 stage, and the columnar endoderm cells, lining the gastrula cavily, derived from the ciliated cells of the earlier stage ; the Inrva is Tixed 

 by the ainrehoid cells on the side on which the original mouth opening is situated : G, H, the young sponge shortly after the development 

 of the spicules; 6, view from the side; H, view from the free extremity, os, osciilum : ec, ectoderm: en, endoderm, composed of ciliated 

 cells ; cs, segmentation cavity. The terminal osculum and lateral pores are represented as oval dark spaces in ti, 11. 



like epithelial cells ; pores appear in the walls, and put the flagellated chambers in communication 

 with the exterior (Fig. 12, E), and subsequently an osculum is formed. By a folding of the outer wall, 

 and other changes, a simple excurrent and incurrent canal system is produced (Fig. 12, F). 



The amphiblastula (Fig. 13, D) is a hollow sphere, one hemisphere formed of a single layer of 

 small, transparent, cylindrical, flagellated cells (epiblast), the other of large, granular, rounded, and not 

 flagellated cells (hypoblast). It results from a metamorphosis of the blastula (Fig. 13, c), the cells of 

 which, for the greater part, are converted into the small flagellated cells, while a few at the base become 

 granular, and by multiplication produce the larger rounded cells. The next step in the development 

 of the amphiblastula is most interesting and important : the flagellated cells become gradually with- 

 drawn or invaginated into the hemisphere of granular cells ; the central cavity of the amphiblastula is 

 thus obliterated, and replaced by another surrounded by the flagellated cells (Fig. 13, E). The embryo 

 now resembles a gastrula of one of the higher animals, consisting, as it does, of a sac with a two- 

 layered wall, and a central cavity communicating with the exterior by a mouth. It soon settles, 

 mouth downwards, on some foreign object (Fig. 13, F), the outer granular cells become the ectoderm, 

 with its usual characters, the inner flagellated cells the endoderm, and a mesodermic layer appears 



