DEVELOPMENT PRIOR TO LAYIXG 53 



The first step in the process of gastrulation, or formation of 

 the primary entodeiTn, is a thinning of the bla stoderm, which 

 begins sUghtly post erior to tj ie^^center and rapidh^ involves a 

 sector of the poste rior thi rd 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 a single 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, e.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 (cf. 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). 



