EXPLANATION OF PLATES. 



PLATE 1. 



1 Injection of the vascular system of an embryo pig of 23 somites, which measured 7 mm. when fresh and 6.2 nun. 

 after fixation and dehydration. The injection is nearly complete and shows especially the primitive relations 

 of the vasa primitiva rhomhencephali at the stage when it serves as a vein for the forebrain and midbrain 

 and as a capillary plexus for the hindbrain. X3S. A. 1., artery of the first interspace; al., allantois; a. si., 

 a. subintestinalis; b. c., bulbus cordis; 1., liver; pi., plexus on the spinal cord; s. v., sinus venosus; /. a., 

 truncus arteriosus; c. c. a., v. cardinalis anterior; r. c. I., v. cardinalis lateralis; v. c. p., v. cardinalis poste- 

 rior; v. u., v. urnbilicalis; va. p. r., vasa primitiva rhombencephali; ren. c., ventriculus cordis. 



2. Partial injection of the vessels of a chick of 12 somites. The needle was introduced into one of the omphalo- 



mesenteric veins near the heart. The transverse lines show the position of the interspaces. The sections 

 shown in text-figures 1, 2, and 3 are from a chick of the same stage which was completely injected. X54. 

 Me., mesencephalon at the level of the section shown in figure 1 ; v. om., v. omphalo-mesenterica. 



3. Injection of the heart and the cephalic aorta 1 , both dorsal and ventral, in a chick of 9 somites. The needle was 



introduced into the dorsal aorta opposite the somites. X04. Ao. d. c., aorta dorsalis cephalica; no. v. c., 

 aorta ventralis cephalica ; /(., heart; me., mesencephalon; v. om., v. omphalo-mesenterica. 



PLATE 2. 



1. Partial injection of the vessels of a chick of 14 somites. The needle was introduced into the dorsal aorta opposite 



the somites. The vascular plexus on the mesencephalon is not injected, though it is present at this stage, as 

 is shown in text-figure 1. X100. A. so., artery of the somatopleure; d. C., duct of Cuvier just before it has 

 connected with the omphalo-mesenteric vein; me., mesencephalon; v. c., v. cardinalis communis, that is, 

 before it has an anterior and a posterior division; v. so., vein of the somatopleure; r. t., v. transversa of the 

 first interspace; va. p. r., vasa primitiva rhombencephali. 



2. Injection of the blood-vessels of a chick of 16 somites, to show the relation of the primitive vessel of the rhomben- 



cephalon to the primitive cerebral vein, on the one hand, and to the anterior cardinal vein, on the other. 

 This is the stage before the vena capitis prima is completed. X58. D. C., ductus Cuvieri; v. c. a., 

 v. cardinalis anterior; v. c. p., v. cardinalis posterior; v. ce. p., v. cerebralis primitiva, which will become 

 the cephalic division of the v. capitis prima; v. I., v. transversa of the first interspace; va. p. r., vasa 

 primitiva rhombencephali. 



3. Transverse section of an injected chick of 30 somites, after 52 j hours of incubation, passing through the twenty- 



first interspace. The section is from the same series as figure 4 on the same plate, figures 2 and 3 on plate 3, 

 and figure 2 on plate 4. Figure 4 on plate 3 is from another series. This section is to show the beginning of 

 the spinal arteries as they show in the tenth to the seventeenth interspaces on plate 3, figure 1. The section 

 is below the level of the omphalo-mesenteric arteries. It is 50 n thick and is unstained. X140. Ao., aorta; 

 v. c. p., v. cardinalis posterior; r. om. p., v. omphalo-mesenterica posterior; W. d., Wolffian duct. 



4. Transverse section of an injected chick of 30 somites, after 52^ hours of incubation, passing tlirough the twenty- 



seventh interspace. The section is to show the relative position of the direct arteries to the posterior cardinal 

 vein and the arteries of the somatopleure. The section is below the level of the omphalo-mesenteric arteries 

 and is in the region of the posterior limb-bud. It is 50 n thick and is unstained. X 140. Ao., aorta; v. c. p., 

 v. cardinalis posterior; r. om. p., v. omphalo-mesenterica posterior; W. d., Wolffian duct. 



PLATE 3. 



1 . The cardinal veins from an injected cluck of 25 somites, to show the method of origin of the spinal arteries. X 106. 



A. 3 and a. 18, arteries of the third and eighteenth interspaces; rf C., ductus Cuvieri; pi., plexus on the 

 spinal cord; i'. c. a., v. cardinalis anterior; v. c. p., v. cardinalis posterior; v. I., v. transversa of the first inter- 

 space; va. p. r., vasa primitiva rhombencephali. 



2. Transverse section of an injected chick of 30 somites, after 52^ hours of incubation, passing through the fifteenth 



interspace. The section is to show a spinal artery like that of the seventh interspace of plate 3, figure 1; 

 il is above the level of the omphalo-mesenteric arteries and slums the posterior omphalo-mesenterie veins 

 on either side. The section is 50 M thick and is unstained. X140. .Ao., aorta; c. c. p., v. cardinalis posterior; 

 v. om. />., v. omphalo-mesenterica posterior; W. d., Wolffian duct. 



3. Transverse section of an injected chick of 30 somites, after 52 hours of incubation, passing tlirough the sixteenth 



somite. This is the next section in the series below that of plate 3, figure 2. It shows a direct dorso-lateral 

 artery to the posterior cardinal vein. The section is 50 M thick and is unstained. X140. Ao., aorta; 

 v. c. p., v. cardinalis posterior; i>. om. p., v. omphalo-mesenterica posterior; W. d., Wolffian duct. 



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