DEVELOPMENT OF TKICHOGUAMMA EVANESCENS 177 
Hypercliromatic nuclei are known to occur in nurse cells 
of insects, in various cells of vertebrate foetal membranes, 
and in many tissues concerned in nourishment, and where 
these nuclei do not become noticeably hypercliromatic, they 
generally hypertrophy. 
The extruded granules are, therefore, to be regarded as 
superfluous chromatin, which has arisen through the peculiar 
conditions to which the blastoderm nuclei are exposed. 
Formation of the Gferm Layers. — In view of the- 
fact that the egg of Trichogramma is not provided with yolk 
the formation of the germ layers is of great interest, for the 
yolk profoundly alters the organogeny in the usual hexapod 
development. That one would receive a faithful representa- 
tion of the ancestral mode of development of the insect, 
from the case of Trichgramma is too much to expect, because 
the method of development, though primitive in some 
respects, is overshadowed by the effects of the parasitic mode 
of life. The blastoderm stage is without doubt quite 
normal, and except for minor nuclear phenomena differs not 
at all from that of the host or of Miastor (3), but the events 
leading to the formation of the endoderm are interesting. 
That the progress figured in PI. 11, figs. 14 and 19, is one of 
gastrulation one hardly doubts. In the case of Polygnotus 
minutus Marchal (7) describes how the embryo is formed 
by a complete invagination of one side of the hollow blastula, 
to form a two-layered gastrula. The method of gastrulation. 
in the parasite treated in this paper is somewhat less distinct 
than in the case of Polygnotus, and before the process is 
far advanced a secondary insinking of other peripheral 
nuclei almost completely obscures it (compare PI. 11, figs. 14 
and 15.) 
The manner in which the endoderm is formed in Tricho- 
gramma is of very considerable interest in view of the dis- 
cussions which have been caused by the different opinions 
expressed by several authors (Dohrn, Kowalevsky, and 
Granin (7) ), but it is not intended here to review their widely 
different suggestions in the light shed by Trichogramma. 
