DEVELOPMENT PRIOR TO LAYING 53 



the primary entoderm, is a thinning of the blastoderm, wliich 

 begins sHghtly posterior to the center and rapidly involves a 

 sector of the posterior third of the blastoderm. This process 

 occurs between twenty and thirty-one hours after fertilization. 

 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 

 Ijlastoderm thirty-one hours after fertilization. It will be ob- 

 served that the anterior portion of the blastoderm is several 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 posterior 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 blas- 

 toderm, and Patterson states that the average diameter twenty- 

 one hours after fertilization is 1.915 mm. and 2.573 mm. ten 

 hours later. The thinning also involves enlargement 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). 



