The Germ-Cells, Fertilization. and Early Development of Grantia (Sycon) compressa*

Abstract

Choanocyte or Collar-Cell. The nuclear background contains a fairly dense basophil substance, which is either a dense linin network or a chromophil karyoplasm-more probably the latter (Pl. 19. fig. 1). The collar of the choanocyte has two thickenings, an inner and an outer; the latter one is very well marked; both thickenings are more easily seen in the partly withdrawn collar (Pl. 19. fig. 1, B). The collar consists of delicate protoplasm which is best preserved by prolonged osmication; the collar-cell is often drawn out into two regions, an outer “neck”and an inner “base”, containing all the granules and the nucleus (Pl. 19. fig. 4); the Golgi apparatus and centrosome generally lie in the “neck,” other granules in the “base”. Collar-cells contain three cell-elements other than the nucleus: the Golgi apparatus (and centrosome), the mitochondria, and metaplasmic store-granuls. The mitochondria are irregular, but fine, and lie around the nucleus (PI. 19. fig. 1, M, fig. 3, M.) The metaplasmic or yolky store granules are much larger than the mitochondria, but lie in the same region. (Pl. 19. figs. 1 and 4, Y). The Golgi apparatus lies in the “neck” of the bottl-shaped cell, either upon the nucleus (Pl. 19. fig. 1, A) or separately (Pl. 19. fig. 1, B). In certain definite cases the centrosome is seen to lie inside the Golgi apparatus (Pl. 19. fig. 1, A) and the flagellum originates from the centrosome.