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 
larva, 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 larva 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) are 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 cf 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. 
