262 CLA YPOLE. [Vou. XIV. 
connection with the membranes. In Oniscus the cells migrate 
bodily instead of simply elongating, and form a cellular cap 
instead of a membranous one. In Xiphidium the cellular 
envelope is completed and entirely encloses the embryo. 
Wheeler’s suggestion is that the organ he calls the “indusium ”’ 
had probably lost its original function, and was degenerating 
and varying in consequence; accidentally acquiring a new 
value, it was reconstructed for its new use as an embryonic 
envelope. In Oniscus this process of reconstruction is not 
yet completed, and in Anurida barely begun. There is an in- 
teresting suggestion in Wheeler’s ('93) mention of the embryonic 
sucking disc in Clepsine. A complete series may possibly be 
made between this disc, the organ as found in Anurida and 
the phyllopod cervical gland which actually functions as a 
sucker, and is regarded by Miiller (64) and Grobben ('79) as 
the homologue of the ‘dorsal organ”’ of the Amphipoda. In 
this case the power of adhesion that still belongs to the pre- 
cephalic organ in Anurida is possibly a remnant of its former 
function. The gradual prolongation of embryonic life causes 
the young to hatch in a more mature stage, and need for the 
adhesive disc of the immature larva is lost. 
Embryo Formation. 
After the separation of the second membrane the formation 
of the ventral plate or germ band begins. On surface views it 
first appears as a narrow band passing round the egg in such 
a way that it nearly encircles it, the precephalic organ being 
the separating mass. The head of the embryo lies on one side 
of it, and by crossing the organ the tail is found at the opposite 
side (Pl. XXIII, Fig. 41). Almost immediately the outlines of 
the embryo can be distinguished, the different parts being laid 
down successively from the head backwards. The mesodermic 
somites indicating the future segments of the body appear, and 
almost at once the appendages of the different parts. In as 
early a stage as that shown in Pl. XXIII, Fig. 41, the begin- 
nings of the antennae, mandibles, maxillae, and thoracic legs 
are evident, and Pl. XXIII, Fig. 40, shows an added pair of 
