174 NOTES AND MEMORANDA. 



achromatic set composed wholly of flint glass or any other suitable 

 refracting substance." 



In connection with this subject, we may refer to a paper read by 

 Professor Stokes at theEoyal Society,* in which he describes an easy, 

 and at the same time accurate, method of determining the ratio of the 

 dispersions of glasses intended for objectives — a method depending on 

 the achromatizing of one prism by another. 



Development of Spongilla fluviatilis. — Professor Ganin, of War- 

 saw, has undertaken some investigations to decide the following mor- 

 phological questions: — Does the gastrula stage exist in the develop- 

 mental history of Spongilla f and if it exists, what is its ontogenetic 

 significance? In what way are the germ-layers formed, and in what 

 relation do they stand to the adult structures of the sponge ? Does 

 the so-called syncytium of Haeckel exist in Spongilla ? Is the ento- 

 derm in sponges confined to the so-called ciliated chambers and 

 their homologues (radial tubes of the Sycones) ? 



In opposition to Haeckel's views on this last question, it is stated 

 by F. E. Schulze, Barrois, and Metschnikof, that the ciliated chambers 

 (radial tubes) do not open into the digestive cavities, but into cavities 

 or canals which are lined with a continuation of the ectoderm. If 

 this last view of the morphological import of the internal cavities 

 of sponges is correct, the homology of their canal system with the 

 gastro-vascular system of the Coelenterata disappears, and the place of 

 sponges in the latter group has still to be demonstrated. 



The answer to all these questions will be found in the author's 

 forthcoming work, ' Contributions to the Anatomy and Developmental 

 History of Sponges,' of which the following is a brief summary of the 

 more important results. 



The ovum of Spongilla undergoes a complete segmentation into 

 equal-sized blastomeres, a solid globular mass of cells — the so-called 

 morula — being produced. The peripheral cells of the embryo then 

 begin to multiply more quickly, and thus become distinguished from 

 the larger and darker cells of the inner mass. In this way the two 

 primary germ-membranes, the (primitive) ectoderm and entoderm, are 

 differentiated from each other. Simultaneously with the commence- 

 ment of this separation, a cavity is hollowed out in the interior of the 

 central mass of entoderm, as the result of the disaggregation and 

 dissolution of its cells. This gastric cavity never opens during the 

 whole period of embryonic development, or during the free existence of 

 the larva. The morula stage passes first into the so-called plano-gastrula 

 or planula stage — a larva of regular oval form, with large internal 

 cavity, and without any external opening. The inner series of the cells 

 of the primitive thick entoderm mass alter their form and structui-e 

 at an early period, and become the actual entoderm of the adult. The 

 remainder of the entoderm mass forms the mesoderm of the larva. 

 This consists of several rows of dark granular cells, filled with rounded 

 yolk-spheres. The spiculae of the skeleton begin to develop very 

 early in the interior of the mesoderm cells. The body of the ovoid 



* 'Proc. Roy. Soc.,' vol. xxvii. (1878) p. 485. 



