634 H. GRAHAM CANNON 
invagination proceeds these lips gradually approach one another 
(figs. 7 and 8) and thereby tend to enclose a space which 
sometimes persists for a short time as a small cavity (fig. 5). 
The lips ultimately fuse (fig. 6), s9 that the primordium of the 
gonads comes to lie completely internally. With the closure 
of this invagination the passage of the ectomesodermal cells 
into the interior stops in this region. 
At the time of invagination the number of cells constituting 
the genital rudimert is about ten, but there seems to be no 
constant number. Cell divisions among these cells were found 
but rarely. Vollmer (14) states: ‘ Teilungsfiguren habe ich 
aber memals in der Gonadenanlage nachweisen kinnen’, 
but in §. vetulus the number of cells m the genital rudi- 
ment most certainly increases by cell division from about four 
at its earlest apparent differentiation to about ten at its 
invagination. 
While these changes have been taking place at the posterior 
end of the ventral mass the formation of the * mesendoderm ’ 
has commenced at the anterior end. The original compact 
yolky cells at this end apparently separate into two parts— 
an inner mass of cells which spread themselves as mesodermal 
cells over the anterior part of the blastoderm conspicuously 
in the region of the ‘ Scheitelplatten ’, and an outer region 
which remains as part of the blastoderm. In the centre of this 
region, that is about midway betweer the genital rudiment 
and the level of the ‘ Scheitelplatten ’, the mesendoderm makes 
its first appearance as a group of tall, compact, comparatively 
non-yolky cells in the blastoderm (fig. 7). The nuclei of these 
mesendoderm cells show at first no difference from those of the 
blastoderm cells, but later, as the mesendoderm mass grows, 
the nuclei are seen to be nearly double as large as the blastoderm 
nuclei, with conspicuous nucleoh. This enlargement can be 
seen to take place as the cells pass inwards. The mass enlarges 
and its posterior end pushes its way backward in the median 
plane (fig. 8). The area of origin, which may be termed the 
blastozone, is marked a little later by a depression from which 
later grows the stomodaeum. 
