DEVELOPMKNT OF TRIUHOGRAMMA EVANESCENS. 167 
takes part in the formation of no organ except the mid-gut, 
but, as I have already pointed out, some of the cells forming 
the mesenteron may conceivably be of another origin, 
namely from nuclei which sporadically wander in from the 
periphery on other parts of the surface of the embryo 
(PI. 11, fig. 15, XT.). From the first the nuclei destined to 
form the mid-gut are conspicuous by their large size and 
rapid growth. The lumen of the mesenteron appears just 
after the stage drawn in PI. 11, figs. 27 and 28. It seems to 
be formed by an internal delamination of the solid endo- 
dermal cell mass in some cases, but in others it looks as if, 
during growth, the ring of cells, gradually enlarging, left a 
lumen in their centre, just as the lumen is known to appear 
in an ordinary duct. In any case there is always a residuum 
left in the developing lumen (PI. 12, figs. 27 and 28). After 
the endodermal cells have grouped themselves as shown in 
PL 11, fig. 24, the proctodaeum aiid stomodaeum begin to be 
quite recognisable ; and there is no doubt that the latter is 
formed by a regular invagination (PI. 12, fig. 31, ST.). The 
manner in which the proctodaeum is formed is a little more 
doubtful. In the case of the stomodaeum the invagination is 
normal (PI. 11, fig. 27). The iupushing cells meet the 
roughly disposed endoderm cells, and when the final dis- 
solving out and disintegration of that part of the embryo 
which forms the body-cavity takes place the connection 
between the stomodasal and mesenteron cells remains unbroken. 
The same thing applies to a region where the proctodaeum is 
formed, but it is difficult to be sure of a true invagination 
such as occurs with the stomodaeum. The latter is formed of 
much smaller cells than the proctodaeum, and is longer, while 
the demarcation between mesenteron and proctodaeum is 
quite indistinct. In PI. 12, fig. 30, which is a horizontal 
section of the front region of a larva of the same age as that 
drawn in PI. 12, fig. 38; the stomodaeum, mouth, and 
mesenteron are shown. In PI. 12, fig. 34, there is a longi- 
tudinal section of the proctodaeum of a somewhat younger 
larva, but it serves to show how short the hind gut is. This 
