260 LIST OF ILLUSTRATIONS. 



PLATE 3 continued. 



FIG. 11. Blastoderm of a chick (No. 174) with 14 somites, incubated for 52 hours and 45 minutes and then grown for 



2 hours and 15 minutes in Locke-Lewis solution having 1.06 per cent of NaCl and only 0.014 per cent of 

 CaClz. It had 12 somites when taken from the shell. It shows the plexus of vessels in the entire area 

 pellucida and demonstrates that the blood-islands in the posterior part of the area pellucida are within the 

 vessels. The interspaces are pale rings and the vessels are a gray plexus, while the dark spots in the plexus 

 are the blood-islands. The heart was beating but there was no circulation, and hence the veins are empty 

 except for a few clumps of red cells, which have developed in them. In the vitelline veins close to the heart 

 the streaks are due to the fact that the interspaces are so close that the endothelium of two adjacent vessels 

 touch and therefore appear as lines. The rectangle covers the area from which two drawings were made, 

 namely figures 23 and 26. Xll. 



FIG. 12. Blastoderm of a chick (No. 145) with 17 somites, incubated for 43 hours and 15 minutes and then grown 

 for 2 hours in Locke-Lewis solution having 1.06 per cent of NaCl. It has blood-vessels throughout the 

 area pellucida. This is the specimen in which the islands were drawn in the living form shown in figure 18 

 from the zone indicated by the line No. 1. It had 17 somites when taken from the shell and the number did 

 not increase. The heart was beating and the circulation had begun, but was not reestablished on the 

 coverslip. It is also the specimen in which the blood of the outer zone of the area opaca has been drawn 

 forward into the veins, and in which a generation of new blood-islands is beginning in this area. X8.6. 



FIG. 13. Blastoderm of a chick (No. 183) with 18 somites, incubated for 52 hours and 45 minutes, and then grown 

 4 hours in Locke-Lewis solution containing 1.06 per cent of NaCl and only 0.014 per cent of CaCk. It had 

 18 somites when taken from the shell and the number did not increase. The heart was beating vigorously 

 and the circulation was reestablished on the coverslip. It shows three areas in which blood-cells are some- 

 what abnormally massed; first, in the anterior veins; second, in the vitelline veins close to the heart; 

 third, opposite the emphalo-mesenteric arteries. It has blood-vessels throughout the area pellucida and 

 shows a large number of blood-islands in the posterior part of the area pellucida. X7.2. 



PLATE 4. 



FIG. 14. Photograph of a section through a blastoderm (No. 144) with no somites to show endodermal blisters. 

 The section passes through the lower part of the primitive streak at approximately the level of the transverse 

 line on plate 1, figure 2. The chick was incubated for 26 hours and 4.5 minutes, was then grown for 2 hours 

 and 10 minutes in Locke-Lewis solution having 1.06 per cent of NaCl, and fixed while a few endodermal 

 blisters were present. There is a small blister on the left side which contains a wandering endodermal cell 

 and a larger empty one on the right side. The mesoderm is in solid undifferentiated masses close to or touch- 

 ing the ectoderm. X60. 



FIG. 15. Photograph of a section through the amnio-cardiac vescicles of a chick (No. 115) with 3 somites which had 

 been incubated for 23 hours and 25 minutes and then grown for 2 hours in Locke-Lewis solution. It had 



3 somites when taken from the shell and the number did not increase. The section is taken at approxi- 

 mately the level of the line across plate 1, figure 4. The section shows an open neural tube. The large, 

 closely packed vescicles characteristic of the development of the amnio-cardiac vescicles are clearly shown, 

 with small clumps of angioblasts opposite the walls of these vescicles. X60. A, angioblasts; C, coelom. 



FIG. 16. Photograph of a section through the first somite from a blastoderm (No. 155) with 5 somites, incubated 

 for 27 hours and 20 minutes and then grown for 2 hours in Locke-Lewis solution having 1.06 per cent of 

 NaCl. It was taken at approximately the level of the transverse line on plate 1, figure 5, which is a specimen 

 of the same stage. The section shows the type of the co?lom, which is characteristic of the middle and pos- 

 terior zones of the area pullucida. In the area pellucida the wide gaps between the vescicles of the ccelom, 

 where there are no cells except the ectoderm and endoderm, are very plain. X60. 



FIG. 17. Photograph of a section of a blastoderm (No. 95) with 11 somites, incubated for 42 hours and 30 minutes 

 and then grown for 2 hours in Locke-Lewis solution. It had 9 somites when taken from the shell. The 

 photograph is given especially to show the position of the drawing on plate 6, figure 28, which is taken within 

 the square. It shows all the processes of formation of the blood-vessels and blood-islands in a single section. 

 At the outer edge of the area opaca are blood-vessels, the sinus marginalis containing free blood-corpuscles. 

 Within the square at the inner border of the area opaca are both blood-islands in a vessel and angioblasts 

 liquefying to form the lumen of a vessel, while in the area pellucida are new clumps of angioblasts. X48. 

 A, angioblasts; /, lumen of the sinus marginalis. 



FIG. 18. Blood-islands in the vessels of the area pellucida of a chick (No. 145) of 17 somites, drawn from the living 

 specimen. It is designed to show as nearly as possible the appearance of the living tissues. The region 

 from which the drawing was taken is shown by the leader No. 1, plate 3, figure 12, from a photograph of the 

 same specimen, but which does not show the exact form of the drawing since the specimen was fixed 2 hours 

 and 15 minutes after the outlines for the drawing were made. The lumina of the vessels is made to appear 

 like ground glass, which represents their actual appearance. The small drawing at the bottom of the figure 

 shows the phase of division of the nuclei, which took place while the drawing was being made. At the top 

 of the figure are two unicellular blood-islands and some free red blood-corpuscles in the lumen of the vessel. On 

 the right side is a large blood-island attached to the endothelium of the vessel. X450. B. i., young blood- 

 island shown in the resting phase, the small drawing below being the same island during division; e., endothe- 

 lium; i., interspace between the vessels as it appears in the living specimen; it represents the mesoderm 

 beneath the vessels, not analyzed with reference to its cells, and is bordered with a rim of endothelium; 

 I., lumen of a vessel. 



