DEVELOPMENT PRIOR TO LAYING as) 
The first step in the process of gastrulation, or formation of 
the primary entoderm, is a thinning of the blastoderm, which 
begins slightly posterior to the center and rapidly involves a 
sector of the posterior third of the blastoderm. This process 
occurs between the twenty-first and tenth hours prior to laying. 
It is due apparently to the gradual rearrangement of the cells 
in asingle layer. A late stage of this process is shown in Figure 
26, which represents a complete longitudinal section through the 
blastoderm ten hours before laying. It will be observed that the 
anterior portion of the blastoderm is many cells thick (26 A), 
but as one passes towards the posterior end the number of layers 
becomes less, and is reduced to a single layer at the extreme pos- 
terior end. Here and there, ¢.g., at X, the arrangement of the 
cells indicates that cells of the lower layer are entering the upper 
layer. It is obvious that such a process must result in increase 
of the diameter of the blastoderm, and Patterson states that the 
average diameter twenty hours prior to laying is 1.915 mm. and 
2.573 mm. ten hours later. The thinning also involves enlarge- 
ment of the segmentation cavity, which may now be known 
as the subgerminal cavity. 
Hand in hand with the thinning out there takes place an 
interruption of the germ-wall at the posterior end, so that in this 
region the margin no longer enters a syncytium but rests directly 
on the yolk (ef. anterior and posterior ends of Fig. 26). 
Figure 27 is a reconstruction of the stage in question. The 
germ-wall, represented by the parallel lines, is absent at the 
posterior end. Here the cells of the blastoderm rest directly 
on the yolk. The sector bounded by this free margin and the 
broken line represents the area of the blastoderm that is 
approximately one cell thick. The figures 2 to 7 indicate 
regions approximately two to seven cells thick. 
Gastrulation begins by an involution or rolling under of the 
free margin, as though the free edge were tucked in beneath the 
blastoderm. The involuted edge then begins to grow forward 
towards the center of the blastoderm, and thus establishes a lower 
layer of cells, the primary entoderm. As soon as this process 
is started the margin of the blastoderm begins to thicken, and 
thus the inner layer of cells (entoderm) and the outer layer of 
cells (ectoderm) are continuous with one another in a marginal 
thickening (Fig. 28). 
