264 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
remainder of the body. These relations are also to be seen in 
figure 27 and in the cross-sections shown in Figures 24 and 25, Fig¬ 
ure 24 being: a section through the extreme anterior end of the ani- 
mal, in which the ectoderm is the only tissue cut, and Figure 25 being 
a section just back of it. The sucking tongue is composed of two 
distinct tissues, an outer epithelium and an inner muscular core 
(Figs. 22, 25, big.). At the stage represented by these figures the 
muscle-fibres are just beginning to differentiate. The hinder sur¬ 
face of this muscle mass is in direct contact with the fused median 
portions of the ganglia allata (Fig. 22). Figures 27 and 28 repre¬ 
sent surface views of an embryo in this stage of development. 
It will be noticed in Figure 22 that a communication between 
the intestine and the proctodeum is established. The growth of the 
proctodeum has thus advanced faster than that of the stomodeum, 
as no communication has yet developed between the stomodeum and 
the forward end of the intestine. The opening between the mid¬ 
intestine and the end-intestine is, however, closed again before the 
birth of the larva and remains so during the larval life of the animal, 
a peculiarity of structure which the larva of Melophagus shares 
with that of the honey-bee. 
In the animal represented by these figures, the germinal band 
has extended over the dorsal side, and the back is thus closed. 
The heart is present in the form of a delicate tube extending from 
the ganglia allata to the hinder end of the body. 
In embryos in this stage of development (and not in any other) 
a pair of deep invaginations appears in the outer ectoderm below 
the mouth (PI. 4 , Figs. 24, 25 and 27, gl. sal.); these are probably 
fundaments of the salivary glands. They are entirely transitory 
structures. 
b. The Involution of the Head. — An important change now 
takes place in the development of the forward end of the body. 
The paired discs, which hitherto have consisted of two ectodermic 
thickenings embracing each a simple, diagonally placed slit (PI. 4 , 
Fig. 26, dsc. ce.), now move forward and towards the median line, 
where the slits finally meet and unite, forming thus a single, trans¬ 
verse, slit-like opening which extends nearly across the embryo. It 
is only the upper or ectal portions of the invaginations, however, 
which thus fuse; their inner or ental portions increase very much 
in volume and depth, though remaining free from each other, 
and extend posteriorly in the body-cavity to the vicinity of the 
