BIOLOGY. 
Sponrj. 15 
Development of the spicules of Tethya prox. lyncurium. All spicules 
arise at first in a single mother-cell or scleroblast, to which other forma¬ 
tive cells may or may not join themselves. (1) The small chiasters of the 
surface arise in differentiated cells of the dermal layer. The scleroblasts 
are spherical or oval, about 15/x in diameter, and characterized by small, 
finely reticulate nuclei, dense evenly granulated protoplasm, and a distinct 
cell-membrane. The first signs of a chiaster are a few radiating beams of 
silica placed asymmetrically on one side of the scleroblast and centred at 
a point near the nucleus. As the rays are formed the granules of the 
scleroblast are absorbed until the whole cell is clear and hyaline. The 
rays of the chiaster continuing to grow out from their point of union at 
the common centre, the scleroblast becomes stretched into an irregular 
form, until it finally gives way, setting free the chiaster, to which it is 
only loosely attached. The scleroblast after completing the spicule has 
now again become a compact cell which takes up stains ; its fate is un¬ 
certain, but probably it becomes an ordinary dermal cell. The larger 
chiasters of the parenchyma are formed in a similar manner, but some¬ 
times, apparently, secretion of silica commences before the division of the 
mother-cells of the scleroblasts is complete, so that the formative cell of the 
chiaster is found to contain two nuclei. (2) The spherasters arise in cells 
similar to undifferentiated archseocytes, with a vesicular nucleus contain¬ 
ing a nucleolus, and with a body of indefinite contour, not enclosed in a 
membrane. The first sign of the spheraster is a globular concretion, 
deposited in a vacuole; the globule grows out into rays, which are variable 
in number, but not infrequently exactly four ; several, or at least two 
scleroblasts, each containing a minute tetraxon spicule formed in this way, 
approach each other and unite, and the tetractines fuse to form a single 
spicule with a massive spherical centre and at least eight small rays. 
Further growth [and formation of additional rays?] is the result of deposi¬ 
tion of fresh layers of silica by formative cells of uncertain origin, but 
probably recruited from the parenchyma. When the spicule is complete 
the formative cells probably return to the tissues. (3) The monaxons 
arise like the spherasters in undifferentiated cells, in which a number of 
siliceous concretions appear simultaneously, but soon unite to form a 
small rod [cf. Weltner (62) infra]. Other formative cells become opposed 
to the original scleroblast, so that the large monaxons are covered by cells, 
arranged either as an epithelium or as a syncytial layer. When the 
spicule is complete the formative cells appear to break up and perish. 
[Cf. Butschli (9), supra, n, b, iv, b.] 
The chiasters are to be classed as true microscleres, formed each in a 
single differentiated cell of the dermal layer. The spherasters are to be 
regarded as fused tetractines and classed with the monaxons as macro- 
scleres, each formed at first in an undifferentiated cell to which other 
formative cells become apposed. There is no evidence that the monaxons 
of Tetliya arise by reduction of tetraxons. The link between Tetraxonida 
and Monaxonida is furnished by the development of the spherasters, in 
which the ancestral tetraxons regularly make their appearance; Maas 
(37): (38), pp. 270 & 271. 
The snake-like twisted monaxon megascleres of many fossil Monaxonida 
and Tetractinellida to be distinguished by the prefix op hi-, as ophiamplii- 
strongyle, ophitylostyle ; Schrammen (50), p. 17 footnote. 
Parenchymal spicules of Spongillince [in Bohemian]; Petr (44). The 
nature and occurrence of the siliceous globules or “ pearls ” of Spongillince 
and sponges generally. Those of Spongillince , at least, are to be regarded 
as abnormal formations, referable in part to mechanical or pathological 
causes, and not as malformations of monaxon spicules. Doubt attaches to 
the statement of Maas [(37) supra] that the monaxons of Tethya arise by 
fusion of minute irregular concretions ; the first rudiment of a monaxon is 
