DEVELOPMENT OF TRICHOGRAMMA EYANESCENS. 163 
wider reticulate central part of the egg. After its expulsion 
the widely reticulate central part of the egg (PI. 11, fig. 13) 
disappears (observe PI. 11, figs. 14, 18, and 19). A partial 
vacuolisation may reappear secondarily, as in PI. 11, fig. 15, 
but this is rare. Further description of the fate of this 
extruded mass will be postponed, but it remains for a good 
while lying between the chorion (PL 11, fig. 19, V.M.) 
and the re-formed blastoderm, often becoming much 
flattened. 
The Appearance of the Germ Layers. 
The expulsion of the inner waste mass is a preliminary to 
the incipient formation of the germ layers. On what is later 
the dorsal surface of the embryo a longitudinal groove 
appears, and beneath this groove the regularity of the 
arrangement of the blastoderm nuclei becomes disturbed. On 
a space oocupied by about five or six nuclei broad and seven 
or eight nuclei long a gradual sinking-in begins. In PL 11, 
fig. 18 and 19, the groove is marked I. N. V. and the sinking 
nuclei N. 8. 1. In PL 11, fig. 14, the egg at this period is seen 
in longitudinal section. 
This process is undoubtedly gastrulation, though in view 
of the fact that the representative of the blastula is solid the 
event is somewhat disguised. PL 11, fig*. 19, has a striking 
resemblance to a gastrulating blastula, though there is no 
segmentation cavity or blastoccele. That the groove repre- 
sents the early blastopore ( I.N.V .) I have no doubt, and were 
the depression to become deeper it would form the mesenteron. 
As it happens, this never takes place, the cavity of the gut 
being formed in a different way. 
It has already been shown that about the stage in PL 11, 
figs. 13, 14, the nuclei loose their granule, and the large 
nucleolus becomes placed in the centre of the nucleoplasmic 
zone. By the stage in Pl. 11, fig. 14, this has taken place in 
every nucleus. In this figure the blastopore (I.N.V.) appears 
on the dorsal surface of the anterior end of the egg, but its 
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