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J. BRONTE OATEN BY. 
an examination of PI. 12, fig. 39, that the cells marked MCX., 
which form the mesoderm, appear in a scattered manner, 
being set free by vacuolisations which rise around them as 
the body-cavity is formed. It has already been noticed that 
at this stage many such cells degenerate completely ( X , X., 
PI. 12, fig. 39), and the number which persists in the young 
larva is never constant. 
It is the body cavity cells which most usually exhibit that 
curious resumption of the reticulum of the nucleus shewn in 
PI. 11. figs. 27 and 28, X , and in PL* 12, fig. 34, X. The fate 
of these cells, and the part they play, if any, in histolysis, I 
do not know at present, but at the stage when the larva has 
swallowed up all the yolk in the Donacia egg, they seem few 
in number and much compressed, while their nuclei never 
show the reticulate structure. Some of the loose cells in 
the body-cavity also form muscles, becoming slightly flattened 
under the ectoderm. 
The Germ Cells. 
Trichogramma evanescens is one of those remarkable 
animals where a definite difference can be seen very early to 
exist between germ cells and soma cells; the difference 
between the two lies in the presence of a germ cell determi- 
nant in the former. At the time of segregation we know, 
from the cases of such insects as Chiron omus or Calligraplia, 
the germ cell determinant becomes included in the pole cells 
which later form the gonads, a'nd in some special examples 
pieces of the broken-up determinant can be found in fairly 
late stages of development. The germ cells at the blasto- 
derm stage PL 11, fig. 12, fig. 14, and fig. 15) have been 
described. They have already lost a great deal of affinity 
for any stains, and in bad preparations the nuclei can hardly 
be found. Xot long after the extruded centrally-placed 
nuclei are finally thrown out to the periphery of the egg, the 
germ cells begin to sink inwards. Exactly what causes them 
to move in this manner I am quite at a loss to say, but it is 
easy to watch the eveut taking place. In the fully ferified 
