A. General. 2. Embryology. 5 



Maas ( 2 ) (giving footnote on the question of priority) has confirmed his conclu- 

 sions [cf. Bericht f. 1892 Porif. p 5] that sponges, though with 3 tissue-layers in 

 the adult condition, are essentially two-layered organisms; as stated by himself 

 and Delage the flagellate chambers arise from the flagellate cells of the larva : - 

 the motor elements migrate into the interior of the sponge. The larva in Cor- 

 nacuspongiae shows a columnar epithelium of flagellate cells [designated A below] 

 covering anteriorly and laterally a central mass of cells [B] , solid except in Spon- 

 ffilla. Two main larval types indicate the true classification of the group: - 

 (1) In Desmacidonidae and Axinellidae we find the A cells evenly flagellate and 

 pigmented, chelae where present in the adult appear in the larvae; to this type 

 Aplysilla would appear from Delage's description allied. (2) In Homorhaphidae 

 and Heterorhaphidae pigment is concentrated in a posterior ring, associated with 

 especially large flagellate cells and flagella ; this type leads to Hircinia and other 

 horny sponges possessing a posterior disc of large flagellate cells (indicating a 

 double origin of horny from monaxonid sponges) ; microscleres, even if present in 

 the adult, do not appear in the larvae. - - First Type. Axinella cris tag alii (n. 

 sp.). The larvae (scarlet, 1 2 mm long) are slow in swimming, but show almost 

 worm-like changes in form ; the hinder pole (orange-coloured) has no flagella. 

 With a high power the bodies of the fine flagellate cells (A) are seen as a 

 striatiou of the outer layer ; when the larva is very much extended the distal ends 

 separate, each cell standing free with its flagellum. Sections (preserved in Flemrning's 

 or Hermann's fluid) show the nuclei to be small, disposed as in Esperia [Bericht f. 1892 

 Porif. p 5], larger nuclei occurring in the B cells of the inner mass and the posterior 

 pole. Among A occur slender cells without flagella, but none showing transition to 

 B cells. The cells at the anterior pole of Esperia are possibly metamorphosed flagel- 

 late cells, possibly gland-cells ; not slime-drops, as suggested by Delage. In the firm, 

 gum-like, internal ground-substance lie spicules and two kinds of cells: S l have 

 vesicular nucleus with nucleolus, chromatin in irregular fragments if evident, amoe- 

 boid protoplasm containing large unequal granules : these cells are almost unaltered 

 blastomeres, and practically remain so as the amoeboid and generative 

 cells of the adult; spicule cells, though hyaline, show similar nuclei; B^ are 

 more differentiated, the generally oval nucleus has a uniform chromatin net-work, 

 the protoplasm is homogeneous with fine granulation, aniline stains show a small 

 spherical body in a vacuole-like area recalling a centres ome: these cells lie 

 more or less radially and tangentially, appearing to be the contractile ele- 

 ments; they also form a kind of epithelium at the posterior pole, a few A cells 

 amongst them there suggesting a transition to completely flagellate larvae. There 

 are also in the flagellate epithelium some granular cells with 2? 2 nuclei recalling 

 in disposition a glandular epithelium; they cannot be called epidermiques, 

 but may be identified with the cells so named by Delage. The posterior axial 

 region of the larva contains closely-packed B { cells and acicular spicules ; the latter 

 extend peripherally forwards to surround an anterior axial region almost devoid 

 of cells. Fixation takes place by the anterior third of the larva (normally by 

 the pole) within 24 hours. Metamorphosis. 1 st Stage, Inversion of 

 the layers (1 st half-hour) . The flattened larva shows a transparent amoeboid 

 base surrounding an opaque dark-red central dome, still covered with active fla- 

 gella. These become reduced to mere amoeboid processes , and replaced by flat 

 cells; as in Esperia the impression is that the flagellate cells draw themselves in, 

 and the others grow over them. Clathria coralloides, preserved a few minutes after 

 fixation, showed in section the A layer of small nuclei pressing in irregular un- 

 dulations towards the interior, already separated from the free surface by a layer 

 of B cells with large nuclei. The latter are in continuity with the central mass at 



