480 



Mr. G. Bidder. 



[May 12, 



a pore in Ascetta clathrus is shown in fig. 3. My conclusion on first 

 detecting the form of the normal ectoderm was that the pores were 

 ectoderm cells which had placed themselves in communication with 

 the gastral cavity. I have found, however, where the conditions 

 are most abnormal the ectoderm cells in their usual flask-shaped form 

 apparently inactive ; while, on the other hand, Metschnikoif cells are 

 very frequent in all stages up to the condition in which they are 

 exposed on both gastral and external surfaces and perforated nearly 

 from end to end. I believe that when they have reached this stage 

 they discharge their granules from the external as well as from the 

 gastral surface. The lumen is thus completed, and a perforation 

 through the wall is formed, which, on regeneration of the gastral 

 epithelium, persists as an afferent pore. 



The prosopyle cells of the Heteroccela consist also each of a single 

 perforated, nucleate cell, frequently containing granules (cf. also 

 Polejaeff, ' Chall. Calcarea,' Plate III), and I suggest the hypothesis 

 that primitively the afferent pores of sponges are perforated excretory 

 cells derived from the endoderm, while the ectoderm is a layer of 

 cells excreting constantly from the intercellular jelly, their flask- 

 shaped form having been developed to expose the greatest possible 

 surface to the medium from which the excreted substance is derived. 

 They have been differentiated on the exterior as a covering to the 

 nutritory and reproductory cells of the sponge, in order, by reason of 

 their noxious contents, to form some protection to the naked proto- 

 plasm. 



{May 16, 1892. — I wish to suggest, as a proper subject for expert 

 chemical analysis, the question, how far the substance excreted by 

 Calcarea in the granules of the ectoderm and Metschnikoff cells is 

 allied to the so-called spongin. 



At first sight, such a proposition may appear idle. But Kolliker, 

 0. Schmidt, and many others have recorded in horny sponges cuticle 

 of various degrees of insolubility, up to a point at which it cannot be 

 distinguished in constitution from the horny skeleton with which it 

 stands in connexion. Both from my own observations on an Aplysilla (?) 

 at Naples (bearing a cuticle insoluble in caustic ammonia), and from 

 a study of Schulze's detailed description of the superficial structure, 

 particularly in Eusjpongia, I am persuaded that the ectoderm cells of 

 the horny sponges are of the same form and character as those in the 

 Homoccela. Again, the slimy covering of Halisarca (Merejkovsky) is 

 precisely homologous with the granules excreted by the ectoderm of 

 calcareous sponges. If the cuticle of horny sponges be granular — as 

 Schmidt found it in Esperia, tunicata — these three products are 

 morphologically identical, and it is not unreasonable to inquire if 

 they are chemically related. 



Further, I cannot find the adequate arguments that have led to the 



