LIST OF ILLUSTRATIONS. 

 PLATE 1. 



FIG. 2. Blastoderm of a chick (No. 141) with 1 somite, incubated for 28 hours and then grown for 22 minutes in 

 Locke-Lewis solution in which there was 1.06 per cent of NaCl. It had no somites when taken from the 

 shell. The specimen is viewed from the endodermal surface, as is the case for all of the photographs on 

 plates 1, 2, and 3. The lines of the endodermal blisters in the posterior part of the area pellucida on the 

 right side have been retouched on the photograph. The line across the figure is approximately the level of 

 the section shown on plate 4, figure 14, from a chick of about the same stage and passes through the endo- 

 dermal blisters. X24. S. 1, first somite. 



FIG. 3. Blastoderm of a chick (No. 212) with 2 somites, incubated for 25 hours and then grown for 3 hours and 45 

 minutes in Locke-Lewis solution containing 1.05 per cent NaCl and only 0.014 per cent of CaCl 2 . If had no 

 somites when taken from the shell; 2 are shown faintly in the photograph. X30. A., angioblasts; S. I, 

 first somite. 



FIG. 4. Blastoderm of a chick (No. 201) with 4 somites, incubated for 26 hours and 40 minutes and then grown for 

 35 minutes in Locke-Lewis solution having 1.06 per cent NaCl and only 0.025 per cent of KC1. It had 3 

 somites when taken from the shell. The more anterior line across the figure is approximately the level of tin- 

 section shown on plate 4, figure 15, and passes through the large vescicles of the mesoderm characteristic of 

 the amnio-cardiac vescicles. The more posterior line shows the level of the section on plate 6, figure 29. X24. 



FIG. 5. Blastoderm of a chick (No. 198) with 5 somites, incubated for 26 hours and then grown for an hour in Locke- 

 Lewis solution containing 1.06 per cent of NaCl and only 0.025 per cent of KC1. It had 4 somites when taken 

 from the shell. The line across the figure shows the level of the section shown on plate 4, figure 16, from 

 another specimen of about the same stage. It passes through the delicate network of the coelom character- 

 istic of the middle and posterior part of the area pellucida. The square on the right side shows the zone which 

 has been drawn for plate 6, figure 27, to show the appearance of angioblasts against the delicate network of 

 mesoderm characteristic of the coelom. On the left side of the photograph there is a large amount of free 

 yolk. Stained with hematoxylin alone. X18. 



PLATE 2. 



FIG. 6. Blastoderm of a chick (No. 213) with 5 somites, incubated for 25 hours and 10 minutes and then grown for 

 3 hours and 45 minutes in Locke-Lewis solution containing 1.05 per cent NaCl and only 0.014 per cent of 

 CaCk. It had 3 somites when taken from the shell. It shows especially well four zones in connection with 

 the vascular system: (1) an outer zone of the area opaca, indicated by the leader No. 1, in which all of the 

 angioblasts have become vessels; (2) an inner zone of the area opaca, indicated by the leader No. 2, in which 

 the angioblasts are still solid; (3) an outer zone of the area pellucida with solid angioblasts; (4) an axial zone 

 of dense mesoderm. The rectangle on the right side shows the area which was drawn for plate 5, figure 22, 

 inclosing the transition between the outer and inner zones of the area opaca. X 18. 



FIG. 7. Blastoderm of a chick (No. 238) with 7 somites, incubated for 29 hours and 30 minutes, and then grown for 

 35 minutes in Locke-Lewis solution in which there was 1.06 per cent of NaCl. It had 6 somites when taken 

 from the shell. It shows a number of clumps of quite isolated angioblasts in the posterior part of the area 

 pellucida, and over the amnio-cardiac vescicles there are the long, slender chains of angioblasts characteristic 

 of that region. They have been retouched in the photograph. X15. 



FIG. 8. Blastoderm of a chick (No. 177) with 11 somites incubated for 52 hours and 10 minutes and then grown for 

 3 hours and 35 minutes in Locke-Lewis solution containing 1.06 per cent of NaCl and 0.052 per cent of KC1. 

 It had 10 somites when taken from the shell and the heart was just twitching along the outer border. It 

 shows masses of solid angioblasts in the posterior part of the area pellucida which are in the phase of division. 

 Contrary to the usual position, the heart projects to the left side of the embryo, right side of the photograph. 

 The rectangle indicates the position of plate 5, figure 24. XI 5. 



FIG. 9. Blastoderm of a chick (No. 151) with 11 somites, incubated for 42 hours and 40 minutes and then grown for 

 3 hours and 40 minutes in Locke-Lewis solution in which there was 1.06 per cent of NaCl. It had 10 somites 

 when taken from the shell and the heart was just twitching. The specimen shows an abnormal form of the 

 brain and there are two large defects in the mesoderm, one on the right side and the other at the posterior 

 end. It is especially fine for the plexus of solid angioblasts in the posterior part of the area pellucida; in 

 fact shows the maximum amount of angioblasts in any of my specimens. It also shows the outer and inner 

 zones of the area opaca especially well, for the vessels of the outer zone are packed with blood-cells and the 

 angioblasts of the inner zone have just liquefied. The line across the figure shows the level of plate 4, 

 figure 17. X17. 



PLATE 3. 



FIG. 10. Blastoderm of a chick (No. 150) with 14 somites, incubated for 42 hours and then grown for 2 hours in 

 Locke-Lewis solution in which there was 1.06 per cent of NaCl. It had 11 or possibly 12 somites when taken 

 from the shell and now the fourteenth is just indicated in the undifferentiated mesoderm. From this stage 

 on the number of somites anterior to the cardiac fold can not be made out for certain in the living chick and 

 records should state the number posterior to the cardiac fold. The heart was beating but there was no circu- 

 lation. The photograph shows four zones in connection with the vascular system, in a transverse line just 

 posterior to the dilated end of the spinal cord; an outer zone in the area opaca in which the vessels are packed 

 with free red blood-corpuscles; an inner zone in the area opaca in which there are blood-vessels containing 

 blood islands; an outer zone of the area pellucida with solid angioblasts; and an axial zone in which there are 

 more delicate angioblasts which do not show at this magnification. 13 X. 



259 



