DEVELOPMENT OF SIMOCEPHALUS 635 
In its backward growth the mesendoderm comes up against 
the primordium of the gonads. There is no strict relation 
between the times of mesendoderm formation and of the 
invagination of the genital rudiment--—sometimes the latter 
is completely internal before the mesendoderm begins to 
grow posteriorly. The mesendoderm pushes its way under- 
neath the genital rudiment between this and the blastoderm, 
which may now be called ectoderm, so that the genital rudi- 
ment comes to lie on the dorsal side of the mesendoderm 
(fig. 9). 
During the formation of the mesendoderm, mesoderm cells 
are formed at the periphery of the blastozone, most con- 
spicuously at the anterior and lateral borders, the posterior 
border being obscured by the backwardly-growing mesendo- 
derm. The mesoderm at this stage is grouped, in the posterior 
portion of the embryo, ventro-laterally, while in the anterior 
part it extends dorsally, covering the ‘ Scheitelplatten’. 
When the mesendoderm has finished its backward growth 
it is a very clearly defined mass, and is sharply separated from 
the lateral mesoderm, as can be seen in fig. 11. It now begins 
to flatten out, and its lateral borders cease to be sharply cut 
off from the neighbouring mesoderm. Ultimately the whole 
of the mesoderm and mesendoderm form one flat plate of cells 
lining the inside of the ventral ectoderm. While this fusion 
is taking place the nuclei of the mesoderm and mesendoderm 
cells become smaller, so as to be indistinguishable from those 
of the ectoderm. From this plate of cells in the median plane 
a solid rod of cells separates off, which is the endoderm (fig. 10). 
At this stage the rudiments of the second antennae are already 
showing. Much later, when the large stomodaeum and the 
smaller proctodaeum have grown in from the ectoderm, this 
solid rod acquires a lumen, 
At the time of separation of the endoderm the genital rudi- 
ment still exists in the ventral part of the embryo lying on the 
gut, as a mass of yolky cells with very large nuclei showing 
the same characteristics as the original primordial germ-cells 
of the ventral mass. 
