DESCRIPTION OF EPIIYDATJA BLEMBINGfA. 87 



columnar epithelium round the central mass. This sheet 

 secretes a cuticle round the central mass, and its cells form 

 the amphidiscs. Subsequently the club-shaped cells migrate 

 outwards, and secrete a second cuticle outside the amphi- 

 discs. 



In the same year as Goette, and independently of him, 

 Wierzejski described the development of the gemmule (19). 

 He describes the first rudiment of the gemmule as a group of 

 naked amoeboid cells. He says that the cells of the mother- 

 sponge can migrate to the body of the gemmule and thus 

 increase its volume. The heap of cells brought together 

 through migration from the sponge tissue become differen- 

 tiated into a central mass and a peripheral layer. Shining 

 spherules and granules are deposited in the cells of the 

 central mass, those of the peripheral layer becoming co- 

 lumnar. The amphidiscs are not developed in the peripheral 

 cells, but in the surrounding tissues, and only subsequently 

 migrate to the columnar layer. 



In the year 1892 Zykoff published an account of the de- 

 velopment of the gemmule (21). This account adds little, if 

 anything, to what was known before of the formation of the 

 gemmule. He found, among the ordinary amoeboid cells of 

 the parenchyma, cells which contained a number of refractive 

 granules of a very definite form, which he describes as boat- 

 shaped. He considers the appearance of refractive yolk- 

 substance in a few amoeboid cells of the mesenchyme as the 

 first step in the development of the gemmule. These amoeboid 

 cells have the protoplasmic structure of Fiedler's amoeboid 

 " Fresszellen," but the nuclear structure of his " Nahrzellen." 

 He disagrees with Groette and supports Wierzejski on the 

 question of the origin of the first rudiment of the gennnule. 

 He denies Goette's statement that the flagellated chambers 

 and the epithelial lining of the canals participate in the 

 formation of the gemmule. The rudiment of the gemmule 

 soon becomes differentiated to a central mass of yolk-cells, 

 among which amoeboid cells of the mesenchyme occur, and a 

 peripheral stage which consists of one or two concentric layers 



