A. General. 2. Embryology. 7 



cells [cf. supra p 3 Dendy], the apopyle is wide. Efferent canals lie in the lower 

 part of the sponge, and unite in a vertical cloaca which passes as a tube of tissue 

 through the subdermal space to the exterior. The osculum is changeable in form, 

 its wall containing many layers of contractile epithelial cells, which later form a 

 definite sphincter. The spicule bundles are by now distinctly Axinellid. - - The 

 segmentation of the ovum was followed in My x ilia rosacea: the ovum is a 

 Si cell : it developes, as in Esperia in a follicle with trabeculae, having wahrend 

 dieser Heranbildung eine Anzahl von Zellen resp. deren Material in sich aufge- 

 nommen, das nun als Dotter (stark farbbare 'Pseudozellen') figurirt. The 3 rd 

 fission divides 4 equal blastomeres into 4 macromeres and 4 micromeres ; a farther 

 stage shows a segmentation-cavity surrounded by cells of uniform structure but 

 markedly larger at one pole of the embryo. The smaller divide more quickly and 

 grow over most of the surface, their yolk diminishes and a chromatin nuclear net- 

 work appears, at the sides they form many layers, anteriorly only one. The 

 largest cells of the inner mass (B) lie near the hinder pole, a single large cell is 

 often seen dividing at once in several planes. The protoplasm of the outer cells 

 (A] becomes scanty and free from yolk, the inner cells retain yolk in varying 

 degree. Oxeote spicules appear in cells with Z?, nuclei and hyaline protoplasm; 

 this must have commenced early [cf. infra p 12 Weltner ( 5 )], as full-grown spicules 

 are also present. Rapid division forms an external layer of A in which can be 

 seen neither cell-boundaries nor striation ; B are uniform in structure but differ 

 from each other in size and form. Intercellular jelly appears to attain its vol- 

 ume through modification of the superficial parts of B cells, the embryo increasing 

 in size. Differentiation of B^ now commences, and the A cells begin to assume their 

 characteristic form , the posterior pole is clothed with epithelioid B- 2 '> exception- 

 ally a segmentation-cavity is still present. Chelae are found in the intercellular 

 jelly, their origin was not traced. The free larva differs from that of Aximlla 

 in little but the central mass of chelae in front of a longitudinal bundle of macro- 

 scleres. Escape is through the osculum. Second Ty p e. Gellius varians. Larva 

 white, 1 mm long, pigment-ring brown- violet with brown centre. The inner mass 

 is pointed [mitre-shaped] anteriorly; the external contour is sometimes the same, 

 sometimes evenly convex, leaving an annular space between A and B, which is 

 Ganin's body-cavity. The flagellate epithelium is thin at the apex, but not per- 

 forated, the posterior ring of cells with large flagella have nuclei of the A type, 

 but larger. Besides the inactive B yolk-bearing cells are with similar nuclei 

 amoeboid cells without yolk, round cells with staining protoplasm and uniform 

 granules, and spindle- formed clear cells (spicule cells). -Z? 2 are round, elongated, 

 stellar, and epitheliar. There are no microscleres, the spicules lie as in the larval 

 Axinella. Most of the pigment is in B cells, during metamorphosis it can be 

 followed over the upper surface. The finer pseudopodia of fixation appear to be 

 flagella ; inversion of layers is as in Axinella. The efferent lacunae develope first, 

 while the small nuclei group themselves into chambers; such groups are certainly 

 not amoeboid cells (see Axinella) : there is no subdermal space, but radial afferent 

 canals; the spicules are not at this stage united into meshes. Pachychalina, Cha- 

 linula, and 2 species of Reniera were found to agree closely with Gellius. In Cha- 

 linula fertilis the follicle ends its nutritive functions before segmentation. The 

 ovum shows a strongly staining peripheral layer, suggesting a pseudo-membrane 

 formed after fertilization, the extreme marginal position of the germinal vesicle 

 suggests extrusion of polar bodies. The development is mainly as in the sponges 

 described, but the blastomeres differ less in size (also in Reniera] than in Myxilla, 

 distinction of nuclei appears early, the four-celled stage has a segmentation 

 cavity. Differentiation of B cells begins before all the A cells are cylindrical ; when 



