270 CLA YPOLE. [VoL. XIV. 
the morula, migrates outward and forms a definite layer below 
the ectoderm, the entoderm remains in the interior as one or 
more cell masses and is comparatively unchanged till a late 
period of embryonic development. Finally the cells separate 
and increase in size, and ingulfing yolk arrange themselves to 
form the definitive mesenteron, which contains practically no 
yolk excepting some in an intracellular condition. Whether 
the vitellophags are genetically connected with entoderm is 
not clear, but they very possibly are entoderm cells that early 
assume their digestive powers. That they do not, however, 
take part in the formation of the mesenteron is clear from their 
presence at the time of its formation scattered throughout the 
yolk, recognizable as shrunken degenerating bodies (Fig. 57, y.c.). 
At first sight this process is markedly different from that 
described for other insects; but the differences admit of quite 
ready explanation. The typical process of entoderm formation 
in the other Insecta is by proliferation from two formative 
centres, an oral and anal, that appear at the two ends of an 
elongate blastopore. This process has been demonstrated for 
the Coleoptera by Heider ('g9) and Wheeler (89); in the Diptera 
by Voeltzkow ('s9) and Graber ('s9); in the Hymenoptera by 
Carriére (90); and in the Orthoptera by Wheeler (93). In 
several other forms but a single formative centre is described, 
that one being the anal. From this one or these two centres a 
continuous band is formed by proliferation that finally incloses 
the yolk completely. The great difference between the two 
processes, as described for Anurida and the rest of the Insecta, 
lies in the different disposition of the yolk; in the former case 
it is not inclosed in the mesenteron, and in the latter it is. The 
other variations may be harmonized in the following way: The 
groups of cells that are usually two in number can be consid- 
ered to be the equivalent of the two masses in the higher 
Insecta; the difference in size is marked, the anal mass being 
the one more likely to persist in a recognizable condition. A 
similar difference was observed by Wheeler (93) for Xiphidium, 
where he found the anal centre decidedly larger than the oral. 
The position of the two masses or, as it may be, one rather 
scattered mass just below the precephalic organ may be indica- 
