DEVELOPMENT PRIOR TO LAYING 59 
The margin of invagination is known as the lip of the blasto- 
pore or primitive mouth; the space between this margin and 
the yolk is the blastopore, and the space between the entoderm 
and yolk, derived from part of the subgerminal cavity, is the 
archenteron or primitive intestine. 
Fig. 27. — Diagrammatic reconstruction of the blasto- 
derm of which a longitudinal section is shown in 
Fig. 26. 
C-D., Plane of Fig. 26. 
G. W., Germ-wall. 1, 2, 3, 4, 5, 6, and 7 indicate 
regions of the blastoderm which are approximately from 
1 to 7 cells deep respectively. The broken line around 
1 indicates the region where the blastoderm is approxi- 
mately one cell deep. x 27.2. (After Patterson.) 
The first stage in the formation of the entoderm is interpreted 
as involution of the free margin, and this view is supported by 
the fact, determined by Patterson, that the antero-posterior 
diameter of the blastoderm is shorter than the transverse diameter 
during this process, whereas previously the blastoderm was 
approximately circular. An even stronger support of this view 
is furnished by experiments which demonstrate that injuries to 
the margin made just prior to gastrulation appear later in an 
