STUDIES IN SPICULE FORMATION. 287 
method of preparing microscopic slides of Alcyonium 
digitatum! which I adopted was practically identical with 
Bourne’s as regards the fixation and staining of the specimens, 
but different as regards the region of the colony selected for 
examination and the manner of obtaining thin portions of it. 
Young colonies only were selected (an important point), and 
these were simply fixed by sudden immersion in 1 per cent. 
osmic; they were then thoroughly washed in distilled water, 
and deposited in either Ranvier’s or Weigert’s picro-carmine 
for three hours. When thus effectually stained the colonies 
were again washed and carefully graded up to absolute 
alcohol. The spicules offering too great an obstruction to 
microtome section-cutting, the colonies were transferred 
straight from the absolute alcohol to coco-butter, in which 
substance they were held whilst free-hand sections of them 
were made. ‘These sections were then placed in xylol, and 
finally mounted in Canada balsam. By this method excellent 
preparations were obtained, the nuclei, cell-plasma, and 
organic axes appertaining to the numerous spicules being 
rendered very evident. I occasionally substituted paraffin- 
wax for the coco-butter, but the latter is preferable in many 
respects, and, moreover, is more economical as regards time. 
Before proceeding to the description of the development of 
the spicules in Aleyonium digitatum, it will be as well to 
give a brief account of the general histology, if only merely 
in order to distinguish the spicule-forming cells or scleroblasts 
from neighbouring endoderm-cells, etc.—a distinction that 
has not always been made. ‘The ectoderm, which forms the 
limiting layer of the mesogloea “ consists,’ according to 
Hickson, “of a number cf columnar, spindle-shaped, and 
flask-shaped cells . . . connected at their outer borders, but 
1 These specimens were obtained from Plymouth, and the method above 
described was employed in consequence of the failure to obtain the polyps in 
an expanded condition (as recommended by Bourne) by narcotization. 1 sub- 
sequently fixed the expanded polyps, like Bourne, by rapidly immersing them 
in 1 per cent. osmic; but these, as above explained, did not give good results, 
as compared with those obtained from the free-hand sections of the mass of 
the colony, 
