508 
MR, J. F. BULLAE ON THE DEVELOPMENT 
membranes it was still present, and it then showed no signs of atrophy. In the adult, 
however, I could find no trace of it. 
Organs more or less resembling this have been described in many of the Crustacea, 
and in some other groups of Arthropods. In the Amphipcds* it appears to be con- 
stantly present, and tolerably uniform in structure. It consists of a circular mass of 
cells attached to the inner egg-membrane, in which usually a cavity becomes developed. 
It occupies the same position as in Cymothoci, and disappears probably about the same 
time. 
Among the Isopoda it is much more variable. In Oniscus t it appears first as 
a patch of rounded cells on the dorsal surface of the embryo. At a later stage this 
disappears, and is replaced by a short cellular stalk, from the upper end of which 
a cellular membrane spreads, like a broad saddle, over the back and. sides of the 
embryo. 
In the embryo of Asellus\ the well-known paired leaf-like appendages probably 
represent the dorsal organ. They arise on each side of the body behind the head as 
simple oval masses of cells, which soon increase in size and become three-lobed. A 
cavity is formed in each lobe, which becomes filled with fluid. The three cavities 
eventually unite, and form a single trilobed cavity open to the yolk, in which free cells 
and drops of food-yolk are sometimes found. At a certain period these organs attain 
so great a development that they break through the egg-membranes, and protrude 
freely on each side of the embryo. Diagram B (fig. 1, woodcut), taken from one of 
Van Beneden’s figures, represents these organs in this condition. They occupy nearly 
the same position as the two ends of the elongated cellular band found in the early 
stages of Cymothoa. 
In Praniza § an unpaired dorsal organ is formed in the usual position. It consists 
of a rounded mass of cells, and is in connexion with the inner egg-membrane, which 
at an early stage is evidently cellular, especially in front. 
Among the other groups of Crustacea, Cumci || possesses a similar organ. 
In My sis f a mass of cells appears on each side of the body behind the head, in 
the same position as the leaf-like appendages of Asellus. A hollow appears in each of 
these masses, which becomes filled with fluid, and apparently communicates with the 
yolk. They disappear very early. It is very doubtful whether these organs occur in 
other Podophthalmata. 
In the Spiders,'""" a mass of cells, “cumulus primitif” of Claparede, appears very 
* Meissner, Zeit. fur Wiss. Zool., bd. vi. ; La Valette, Abband. der Naturforsch-Gesell. zu Halle, bd. v. ; 
Fritz Muller, ‘ Facts for Darwin’; Emil Bessels, Jenaische Zeit., bd. v. 
t Bobretzky, Zeit. fur Wiss. Zool., bd. xxiv. 
+ Dohrn, Zeit. fiir Wiss. Zool., bd. xvii. ; E. Van Beneden, Bull. Acad. Roy. Belg., t. xxviii., 1869. 
§ Dohrn, Zeit. fiir Wiss. Zool., bd. xx., 1870. 
|| Dohrn, Jenaische Zeit., bd. v., 1870. 
E. Van Beneden, Bull. Acad. Roy. Belg., t. xxviii., 1869. 
** Claparede, ‘ Recherche sur l’Evolution des Araignees.’ 
