Pseudogastrula Stage in Calcareous Sponges. 101 



fixed. By degrees this food is absorbed and used np, then 

 the invagination of the ciliated cells takes place, and the 

 embryo becomes attached. Hence, in the gastrula stage, the 

 internal granular mass of cells is no longer visible. 



Thus I tliink that the internal granular mass of cells is to 

 be looked upon as food-yolk, and that it has little or 

 nothing to do with the formation of the mesoderm. The 

 latter, as I have alreadj'' shown, appears to be present before 

 the mass of food-yolk is formed, and though hidden by it 

 later on (Figs. 5 and 6), is doubtless still there, and from 

 this tissue the spicules observed by Metschnikoff probably 

 arose. 



My investigations were carried on by means of serial 

 sections of the mother sponge, stained with borax carmine 

 and embedded in paraffin. 



EXPLANATION OF PLATE 1a. 



The following explanation of the lettering applies to all the figures : — 



col. — Collared cells lining the flagellated chambers of the mother sponge. 



e. — Embryo. 



e.c. — Capsule containing the embryo and lined by flattened endothelial cells. 



■ect. — Ectoderm. 



e?id. — Columnar cells of the embryo, which will form the endoderm of 

 the adult. 



f.t/. — Mass of granular food-yolk-coutainiug cells derived from the invagin- 

 ation and proliferation of the granular cells of younger stages. 



gr. — Granular cells of embryo. 



i.e. — Inhalant canal of mother sponge. 



mes. — Commencement of mesoderm of embryo. 



0. — Opening of a flagellated chamber into the gastral cavity of the mDther 

 sponge. 



r.c. — Flagellated chamber of the mother sponge. 



s.m. — Structm-eless membrane, formed of the outer part of the embryo capsule 

 and the remains of the collared cells lining the flagellated chamber 

 of the mother sponge. 



S2). — Spicule of the mother sponge. 



Figure 1. — Section of embryo in late blastosphere stage, lying within the 



embryo capsule in the maternal tissues, between a layer of spicules 



and a flagellated chamber. 

 Figure 2.- -Section of embryo at the commencement of the pseudogastrula 



stage, represented apart from the maternal tissues by which it is 



surrounded. 

 Figure 3. — Portion of section of the mother sponge, showing an embryo 



breaking loose from the embryo-containing cavity into a flagellated 



chamber. (The spicules are represented in blue). 

 Figure 4. — Section of embryo in late pseudogastrula stage, with the 



structureless membrane which adheres to it on its escape from the 



maternal tissues. 

 Figure 5. — Section of a solid embryo found in a flagellated chamber of the 



mother sponge. 

 Figure 6. — Section of the free-swimming, ciliated embryo (diagrammatic). 



