DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



PLATE 3 Continued. 



4. Transverse section of an injected chick of 25 somites after 52 hours of incubation, passing through the seventeenth 



interspace. The section is to show the transition between the stage of (inure 3 of plate 2 and figure 2 of plate 3, 

 in the formation of a spinal artery. It is 50 M thick and is unstained. X 140. Ao., aorta; v. c. p., v. cardi- 

 nalis posterior; 11'. d., Wolffian duct. 



5. Injection of the aorta, the arteries of the pronephros, and the lateral and posterior cardinal veins in an embryo 



pig of 20 somites, from a specimen of the same litter as the one on plate .">, (inure 1 . The specimen is shown 

 for the ventral aspect. X140. A. !>to A. 12, arteries to the spinal cord in the ninth to the twelfth interspaces; 

 ao., aorta; r. c. I., v. cardinalis lateralis; v. c. p., v. cardinalis posterior. 



PLATE 4. 



1 . Injection of the vessels of the head of an embryo pig of 27 somites, measuring 7.1 mm. after fixation and dehydration 



The specimen is from the same litter as the one on plate 5, figure 2, and is to show the completion of the vena 

 capitis prima and its relation to the vasa primitiva rhombeneephali. It shows that the primitive vessel 

 of the hindbrain does not atrophy when the vena capitis prima is completed, but rather develops into a plexus 

 on the hindbrain. X94. A I., atrium; b. c., bulbus cordis; f. H., fretum Halleri; I. a., truncus arteriosus; 

 i'. cap. p. 1, v. capitis prima, first or cerebral segment, which drains the forebrain and midbrain; v. cap. p. 2, 

 v. capitis prima, second segment, which drains the forebrain, the midbrain, and the visceral arches; 

 v. cap. p. 3, v. capitis prima, third segment, which is the anterior cardinal vein; va. p. r., vasa primitiva 

 rhombencephali ; ven. c., ventriculus cordis; ves. a., vesicula auditiva; V, position of the root of the 

 n. trigeminus; VIII, position of the roots of the nn. cochlearis et vestibularis; IX, position of the root of the 

 n. glossopharyngeus. 



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



fifth interspace. The section is to show the diverticula of the dorsal aorta like those of the first and second 

 interspaces in plate 1, figure 2, which give rise to the cardinal veins. The section is 50 M thick and is 

 unstained. X140. Ao., aorta; II'. d., Wolffian duct. 



3. Partial injection of the vessels of an embryo pig of 14 somites, measuring 4 mm. after fixation and dehydration. 



The specimen was injected through the dorsal aorta opposite the somites. X56. A. 9, artery to the spinal 

 cord in the ninth interspace; at., atrium; 6. c., bulbus cordis; /. H., fretum Halleri; s. 1, first somite; 

 s. p., sinus venosus; (. a., truncus arteriosus; v. c. p., v. cardinalis posterior; v. om., v. omphalo-mesenterica ; 

 ven. c., ventriculus cordis; ves. a., vesicula auditiva. 



PLATE 5. 



1. Partial injection of the vessels of an embryo pig of 20 somites, measuring 6 mm. after fixation and dehydration. 



It shows the omphalo-mesenteric arteries, the subintestinal artery and the arteries of the pronephros; X41. 

 A. 9, artery to the spinal cord in the ninth interspace; a. om., a. omphalo-mesenterica; a. si., a. subintestinalis ; 

 al., allantois; b. c., bulbus cordis; s. at., stalk of the allantois; /. a., truncus arteriosus; v. c. I., cardinalis 

 lateralis; ven. c., ventriculus cordis. 



2. Partial injection of the vessels of an embryo pig of 27 somites, measuring 7.1 mm. after fixation and dehydration. 



It shows the general development of the vascular system at the stage when the vena capitis prima is completed. 

 The vessels opposite the hindbrain, both deep and superficial, are extravasated in this embryo and hence they 

 are shown on plate 4, figure 1, from an embryo of the same litter. X51. A. om. d., a. omphalo-mesenterica 

 dextra, the other two omphalo-mesenteric arteries in the figure being on the left side; at., atrium; 6. c., bulbus 

 cordis; il. C., ductus Cuvieri; 1., liver; t. a., truncus arteriosus; v. cap. p. l,v. capitis prima, first or cerebral 

 segment, which drains the forebrain and midbrain; v. cap. p. 3, v. capitis prima, third segment, which is the 

 anterior cardinal vein; c. om., v. omphalo-mesenterica; v. u., v. umbilicalis; ven. c., ventriculus cordis; 

 x., extravasation involving both the vasa primitiva rhombencephali and the vena capitis prima, as can be seen 

 on plate 4, figure 1. 



PLATE 6. 



Injection of the vessels of the head of a chick of 29 somites, to show the origin of the vena capitis prima. The vein 

 extends from the region of the diencephalon to the duct of Cuvier. The injection shows that the vein arises in 

 three segments; the first segment is a true primitive cerebral vein, which drains the forebrain and will soon drain 

 the midbrain; the second segment is an anastomosis between the maxillary, the mandibular, and the other vis- 

 ceral arches and the anterior cardinal vein, and it drains the forebrain, the midbrain, and the visceral arches; 

 the third segment is the anterior cardinal vein, which drains the brain and the visceral arches. X 128. A. b., 

 artery on the rhombencephalon, which at this stage is bilateral and is part of a plexus which will give rise to 

 the basilar artery; a. 3, artery to the medulla in the third interspace; d. C., ductus Cuvieri; !. r. />., 



v. maxillaris primitiva; r. om., v. omphalo-mesenterica; v. t., v. transversa of the first interspace; v. u., plexus 

 in which the v. umbilicalis will arise; va. p. r., vasa primitiva rhombencephali; ves. a., vesicula auditiva; 

 V, position of the root of the n. trigeminus; VIII, position of the roots of the nn. cochlearis et vestibularis. 



