MUSEUM OF COMPARATIVE ZOOLOGY. 133 
nous avons mentionnée.” On page 132 he writes: “On admet que 
ces pieces fourchues sont formées par la division d’un ossicule discoide - 
sur la ligne médiane, et la déviation de chacune des deux moitiés, 
jusqu’a la rencontre de la moitié correspondante du disque voisin a 
laquelle elle se soude.” In these descriptions it seems probable that 
Apostolides refers to the first pair of adambulacral and possibly the 
second pair also. It can hardly be supposed that he refers to the first 
pair of ambulacral, which are later described by me as the spoon-shaped 
plates after Ludwig, since these never have a V-shape. Whether he 
refers in his description to the first ambulacral or the first adambulacral 
(with the second), it may be borne in mind that neither has been formed 
by a division of a “discoid” ossicle, nor are they ever five in number. 
The members of the five pazrs of adambulacral as well as of ambulacral 
originate as ten separate calcifications. In the youngest condition (a 
stage a little younger than Fig. 17) of the growth in which they were 
observed the first pair of adambulacral plates were formed although the 
terminals were very small. I am therefore led to suppose that the first 
pair of adambulacrals appear before the terminals. In this embryo 
(Fig. 17) the pentagonal form is but obscurely indicated. The first pair 
of adambulacral are portions of the “ maxille” of Metschnikoff’s figure 
(PIATV, Fig. 18, ce). 
The first adambulacral plates originate in pairs,about the mouth (Fig. 
15). In early conditions they lie more in those axes which later become 
the interradials, although they arise before the young has passed into 
conditions in which either radii or interradii can be definitely recog- 
nized. In Ludwig’s Fig. 23 they are smaller than the terminals, but in 
some of my preparations they are larger. 
In a later stage (Fig. 17), in which the rays have begun to push out, 
the first pair of adambulacrals assume a more or less irregular crescentic 
form, with concave edges turned to the radii. By a continued growth the 
adradial edge of these plates begins to approximate so that plates of 
adjacent pairs approach each other. This approximation is confined to 
about half their interradial border. The adradial half of the rude cres- 
cent extends to the mouth opening. By the approach of two of these 
first adambulacral plates (ad') which lie in different pairs, a V-shape is 
given to the combination. The plates however are free along the line 
of the interradii. The double network of these plates appears as in the 
radialia (Fig. 18, 7p). 
Of the homology of these plates with the adambulacral there seems no 
doubt. I believe also that Ludwig’s comparison of them to the side 
