262 LIST OF ILLUSTRATIONS. 



PLATE 6 continued. 



FIG. 28. Section through the inner edge of the area opaca of a chick (No. 95) with 1 1 somites, which was incubated 

 for 42 hours and 30 minutes and then grown for 2 hours in Locke-Lewis solution. The chick had 9 somites 

 when taken from the shell, and the eleventh was just appearing when it was fixed. The heart was not beating 

 when the egg was opened, but twitched slightly on the coverslip. The section is approximately at the level 

 of the line across plate 2, figure 9, which is a specimen of about the same stage. A photograph of the section 

 from which the drawing was made is given on plate 4, figure 17, with a square to indicate the zone. It shows 

 the process of liquefaction of angiobla'sts to make vessels, as it can be seen in sections, as well as a blood- 

 island attached to the endothelium of the same vessel. It shows the same processes illustrated in total prep- 

 arations on plate 5 (fig. 23) and plate 6 (fig. 26). X700. A, angioblasts in the process of liquefying, show- 

 ing pycnotic nuclei; b. i., blood-island; c, exoccelom; ec., ectoderm; en., endoderm; I., lumen of vessel; ?es., 

 mesoderm. 



FIG. 29. Section through the posterior part of the area opaca of a chick (No. 148) with 4 somites, which had been 

 incubated for 30 hours and 15 minutes, and then grown for an hour in Locke-Lewis solution having 1.06 per 

 cent of NaCl. It is taken at approximately the level of the lower line on plate 1, figure 4, from a specimen 

 of about the same stage. The drawing shows the differentiation of the primitive mesoderm into the two 

 layers of cells which form the ccelom and the more ventral solid clumps of angioblasts. X620. A, angio- 

 blasts; c, coelom; ec., ectoderm; en. c., endodermal cell which has wandered to a position dorsal to the meso- 

 derm; mes., mesoderm. 



