No. 3-] DEVELOPMENT OF MARINE SPONGES. 319 



point approach those of Delage. That the solid larva turns itself 

 inside out in this fashion is certainly a remarkable phenomenon, 

 and one that calls for abundant evidence. Maas's argument, as 

 far as I can make it out, is that the attached sponge consists 

 of two layers which exactly resemble those of the swimming 

 lar\'a, but that the inner cells in the attached sponge are like 

 the outer cells of the larva ; and, conversely, the outer cells of 

 the attached sponge are like the inner cells of the larva. It 

 seems to me that deductions made from histological similarities 

 of this sort can never be relied on with much confidence. 

 And especially must this be true in a case like the one in hand, 

 where so many of the cells are undergoing histological change. 

 I cannot see that either Delage or Maas proves his case. I 

 have, as has been mentioned, found indications that some of 

 the ectoderm cells of the lan^a migrate into the interior during 

 metamorphosis, but I found no evidence that the ectoderm as 

 a whole does not continue on the surface. 



Maas finds that the subdermal spaces, canals, and chambers 

 arise separately, the spaces and canals as large lacunae in the 

 parenchyma of the sponge. Maas does not believe that the 

 cells which I have called " formative cells " have any share in 

 producing the flagellated chambers. He thinks the chambers 

 are formed from aggregations of the small cells with small 

 nuclei "which in the larva constituted the ciliated epithelium, 

 and during the metamorphosis migrated into the interior " 

 (16, p. 432). Maas believes that the efferent canals (in part, 

 at least) arc formed by similar cells having the same origin. 

 I must confess that all this seems to me highly theoretical, the 

 whole belief resting on a partial histological resemblance 

 between the ciliated cells of the larva and the cells of which 

 the finished or nearly finished chambers are made. As for the 

 canals, I have always seen them formed by cells bearing no 

 resemblance at all to the small slender cells which Maas 

 supposes to be migrated ectoderm cells. In regard to the 

 chambers, I am disposed to believe that the aggregates of 

 small cells described by Maas are not different from those I 

 have described as resulting from the division of larger 

 cells. 



