92 Kle'HARD EVANS. 



the ampliidiscs (PL 4^ fig. 10). While this is going on their 

 inner ends situated between the aniphidiscs become trans- 

 formed to the parenchyma-like substance situated in the 

 mature gemmule between the inner and outer chitinous coats. 

 During the elongation of the columnar cells outwardly the 

 nucleus is carried along. After their inner moiety has been 

 modified and the nucleus has passed to the outer portion they 

 secrete the outer chitinous layer and ultimately break off, and 

 so becoming liberated they pass back again to the sponge 

 tissue (PI. 4, fig. 16a). The nucleus at the close of these 

 changes, as at the beginning, is vesicular. 



The outer chitinous coat is much thinner and less homo- 

 geneous than the inner. In the fully mature gennnule the 

 •greater part of it is lost, so that the outer ends of the 

 ampliidiscs are uncovered. 



(3) The Origin, Migration, and Final Modification 

 of the Scleroblasts, inside which the Ampliidiscs 

 are developed, and their Migration from the Sponge 

 Tissue into the Columnar Layer. — At the outset special 

 emphasis must be laid on the point that incompletely deve- 

 loped ampliidiscs were never seen in the protective coat of 

 the gemmule, whether during the early or later stages. The 

 aniphidiscs situated in the gemmule coat are always fully 

 developed, while in the sponge tissues incompletely developed 

 stages as well as fully developed ones are plentiful. 



The first stage observed in the formation of the ampliidiscs 

 consists of a rod-like structure swollen at both ends (PI. 1, 

 fig. 3, m, and fig. 6, a), in which respect they differ essentially 

 from the young stages of the amphioxea, which are always 

 pointed (PI. 1, fig. 5). Both kinds make their first appear- 

 ance in cells with vesicular nuclei, which soon become trans- 

 formed and become granular, especially in the mother-cells 

 of the ampliidiscs. The next change consists in the deve- 

 lopment of a more or less conical form by the ends of the 

 above-mentioned rods, the cone-shaped end at the same time 

 becoming surrounded by a rim (PI. 1, fig. 6, h). The cone- 

 shaped end, together with its slightly developed rim, ulti- 



