DEVELOPMENT PRIOR TO LAYING 



57 



anterior position in the entoderm (Patterson). But after the 

 margin has thickened the farther extension of the entoderm is 

 due, largely at least, to ingrowth from the marginal thickening. 



Patterson also believes that the thickening of the margin is 

 due not so much to multiplication of cells in situ as to immigration 

 of cells from the sides. This view is also supported by experi- 

 ments. 



C E G 



F H 



FIG. 29. Diagrammatic reconstruction of the blastoderm of a 

 pigeon's egg, 36 hours after fertilization; from the same series as 

 Fig. 28. x 27.2. (After Patterson.) 



E., Invaginated or gut entoderm. O., Margin of overgrowth. 

 PA., Outer margin of pellucid area. R., Margin of invagination 

 (dorsal lip of blastopore). S., Beginning of yolk-sac entoderm. 

 Y., Yolk zone. Z., Zone of junction. 



The arrows at the posterior margin indicate the direction of 

 movement of the halves of the margin. The circles in the pellucid 

 area indicate yolk masses in the segmentation cavity. 



Figure 29 is a reconstruction of a blastoderm in the stage of 

 Fig. 28, that is at the height of gastrulation. The margin of 

 overgrowth (cf. Fig. 28 O) is represented by the area O; the 

 zone of junction by the ruled area Z; the inner portion of the 



