New observations on the fine structure of graptoblasts

Abstract

Graptoblasts are redefined as widenings of the sealed terminal portions of graptolite stolothecae resulting in ovoid bodies composed of an outer fusellar layer, named here as blastotheca, and an inner secondary layer of electron dense material, named the blastocrypt. In their proximal portion a number of graptoblasts display a scar with fuselli stripped and the crassal material of the blastotheca exposed and displaying a rough and dull surface. The boundary of this area, termed the talus, is edged by a distinct escarpment, probably the margin of the preserved part of the blastotheca. The outer wall of the talus is usually abruptly terminated. Only exceptionally is the outer wall conical and gradually widening. Thus the transverse partition between the parental stolotheca and the graptoblast was produced by the blastocrypt without fusellar covering. It seems, therefore, that graptoblasts were most probably lacking any internal portion of their blastothecae which could possibly be compared with the internal parts of a dendroid auto-theca. It is concluded that graptoblasts housed a zooid that never functioned as an autozooid, but suffered arrested development and encystation. The biological role of graptoblasts as dormant bodies is discussed.