218 SCIENCE PROGRESS. 



of the surface. Maas, who does not recognise a distinction 

 between the cellules epidermiques and the other differen- 

 tiated elements of the inner mass, considers that what 

 Delage has seen between the flagellated cells are glandular 

 elements similar to those seen by him in Axinella. 



The next stage in the life history is the fixation of the 

 larva and its metamorphosis into a young sponge. The 

 period of larval life is usually of short duration, and Maas 

 lays it clown as a general rule, that the longer fixation is 

 delayed, the more likely is the subsequent development to 

 become abnormal. Few organisms are so delicate or so 

 liable to abnormalities as sponge larvae, and it may be said 

 that unless they are very carefully tended, anomaly becomes 

 the rule. It is probably owing to this cause that the 

 greatest confusion prevailed among earlier investigators 

 as to the pole of fixation, though "the general statements 

 are in favour of the attachment taking place by the 

 posterior extremity where the granular matter projects " 

 (Balfour, Conip. Embryology). On this point the state- 

 ments of both our authors show a pleasing unanimity, to 

 the effect that normally fixation always takes place by 

 the anterior pole, i.e., by the pole which in all but Aplysilla 

 is ciliated. 



With regard to the metamorphosis, the statements of 

 Maas and Delage are again in agreement as to fundamental 

 points, though differing greatly as to the details of the 

 process. According to both authors, the cell layers com- 

 posing the larva undergo displacements whereby the 

 ciliated cells, which have temporarily lost their flagella, 

 come to be placed in the interior and to be surrounded 

 by the cells of what was formerly the inner mass. The 

 whole organism becomes very much flattened and spread 

 out after fixation, and is at first compact, but spaces soon 

 appear in the parenchyma to form the canal system, round 

 which the various cells group themselves. The ciliated 

 cells give rise to the chambers alone, becoming the collar 

 cells ; the flattened epithelium lining the canals and cover- 

 ing the outer surface, as well as the entire mesoderm, are 

 derived from the inner mass of the larva. 



