DEVELOPMENT PRIOR TO LAYING OW 
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 7 situ as to immigration 
of cells from the sides. This view is also supported by experi- 
ments. 
e 
fo) “os 0 
07, 6, 467.® *~o oo 
9° 
Fic. 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). 8., 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 
