776 DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
§ 12. Development of the spicules and calyx-teeth. 
The spicules of Renilla, as E1sen observed, are of two different forms, and I have 
found these to have an entirely different origin. The large elongated spicules which 
give the colony its beautiful purple colour and stiffen its walls are produced entirely by 
the ectoderm. On the other hand the small transparent oval spicules, which occur in 
small numbers only, are developed in the entoderm alone. 
The former make their appearance soon after the attachment of the larva on each 
side of the middle region of the body near the first pair of buds. They have the 
appearance of delicate transparent rod-like bodies which are at first quite colourless. 
They increase slowly in number, extending backward along the line of the peduncular 
septum on each side. It is only when the formation of the colony is well advanced 
that they acquire a purple colour and begin to extend forwards towards the oral 
extremity and upwards and downwards around the body. They gradually extend 
over the whole area of the body which therefore acquires a delicate purple tint, except 
towards the tip of the peduncle which remains white. The spicules become very 
scanty or quite disappear towards the anterior extremity and assume a peculiar 
arrangement at their upper limit. In each compartment some distance behind the 
bases of the tentacles they arrange themselves in lines radiating backward and sidewise 
from a small central area which ultimately forms the tip of a calyx-tooth (fig. 185, cx.). 
At the same time this region becomes elevated so as to form a low conical prominence 
which in later stages gradually grows out into a hollow pointed diverticulum from the 
chamber, its walls being stiffened by long spicules; this is a calyx-tooth. When the 
crown of tentacles is retracted the tooth lies at the anterior end and forms a hard 
pointed prominence projecting forwards. Calyx-teeth appear on all of the chambers 
except the ventral one where the formation of a tooth is a rare exception. 
In transverse sections the needle-shaped spicules are found to lie in the lower layers 
of the ectoderm outside the lamella, and a study of the smallest spicules shows that 
they are formed in the interior of rounded cells lying in the ectoderm. It is difficult 
to demonstrate the spicule-cells, even in the earliest stages of the spicules, and I have 
never seen them with certainty after the spicules have attained any considerable size. 
They cannot be isolated by teasing, and when in situ it is difficult to distinguish them 
from the surrounding cells. By staining the tissues deeply with eosin the bodies of 
the cells may occasionally be clearly distinguished. Figs. 171* to 171% show different 
forms of the cells containing very young spicules. In some of the cells nuclei appear ; 
in others they are imvisible. The caleareous matter first appears as an irregular 
elongated mass in the protoplasm of the cell and shows to the eye no trace of 
erystualline structure. As a rule there is only a single concretion in a cell, but the 
spicules are occasionally formed from two centres, as in fig. 171%. As the spicule 
increases in size the enclosing layer of protoplasm becomes very thin and I have never 
