268 CLA YPOLE: [VoL. XIV. 
girdling the egg. At first one-layered, the mesoderm early 
shows a further change to two in certain parts which represent 
the mesoblastic somites. Fig. 49 represents a cross-section 
through the germ band just after itsformation. In the middle 
line, under the median ectodermic depression, the mesoderm is 
a single layer of cells; but on each side there are the early 
indications of the somites. The two-layered condition arises 
by migration, and the cavity when present is hollowed out 
afterwards. Subsequent modifications arise by differentiation 
from this primitive condition. | 
Origin and Development of the Entoderm. 
The place and manner of entoderm formation long remained 
in doubt, as the appearance of the mid-gut is delayed till very 
late in embryonic life, and these late stages are difficult to find. 
However, the following facts and explanation were eventually 
determined. By the end of cleavage two definite layers are 
fully established, the ectoderm and mesoderm, as shown in Fig. 
35. There are, moreover, certain cells left in the yolk that 
have never taken part in the formation of either of the two 
layers. Some of these that are spread singly through the yolk 
are evidently yolk cells, and function in the transformation of 
yolk for the nutrition of the embryo (Figs. 35, 37, 39, y.c.). In 
addition to these, however, there are some cells that remain 
grouped in clusters, the whole mass evidently arising by division 
from a single cell or perhaps a few cells. The clusters are 
_ placed above the centre of the egg, using the precephalic organ 
as the pole of the reference axis, and very frequently limited in 
numbers to two (Fig. 35, ez.). One of two things now happens 
to them: they either migrate from the masses and are scattered 
through the yolk in small groups of twos or threes, or else they 
remain unchanged for a considerable length of time. In the 
former case they are difficult to distinguish from the yolk cells, 
but their greater transparency, larger vesicular cell bodies, and 
association in small groups is a sure guide to their identification 
(Pl. XXII, Fig. 41). The yolk cells early acquire a more deeply 
staining nucleus, showing the characteristic increase of chro- 
