DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



115 





arteries gives rise to a plexus which is ventral to the posterior cardinal vein, but 

 which connects with it. The Wolffian duct intervenes between this plexus and 

 the posterior cardinal vein. In figure 1 of plate 5, and figure 1 of plate 1, are 

 given very characteristic views of these lateral arteries to the pronephros, and in 

 figure 5, plate 3, is shown a ventral view of the arteries of the pronephros from a 

 specimen of the same litter as that of plate 5, figure 1, but a little farther developed. 



Figures 1 of plate 5 and 1 of plate 1 show a series of tiny lateral arteries 

 beginning just below the ninth dorsal segmental artery. These arteries are about 

 four to a somite, corresponding to 

 the number of the nephritic tubules, 

 and are connected by a tiny longi- 

 tudinal artery close to the aorta and 

 by a tiny lateral vein. In text- 

 figure 4 is shown a section from a 

 specimen of the litter of plate 1, 



figure 1, passing through about the .^^^^^^^^_ K?&^!&\ 



fourteenth somite, showing an injec- 

 tion of one of these lateral arteries. 

 Its exact position with reference to 

 the developing tubule is, I think, 

 important. This is most clearly 

 recognized from the diagram given 

 by Felix of the development of the 

 nephritic tubules (fig. 561, Keibel 

 and Mall, Manual of Human Em- 

 bryology, page 804) . The stage cor- 

 responds with diagram d of Felix's 

 figure, and the artery passes directly 

 across the curved bowl which makes 

 the neck of the future Malpighian 

 corpuscle. This is the earliest stage 

 of the vessels of the nephritic tubules I have injected. As is seen in text-figure 4, 

 the lateral vein, the vena cardinalis lateralis, lies ventral to the Wolffian duct, 

 while the vena cardinalis posterior lies directly dorsal to the duct. The posterior 

 cardinal vein is plainly shown in text-figure 4, but was not injected so far caudal- 

 ward in any of my series. 



Text-figure 5 gives a very interesting section from the same series as text- 

 figure 4. The level of the section is shown in plate 1, figure 1; it is about halfway 

 between the level of the lowest transverse artery injected and the allantoic arteries. 

 At this level the nephritic tubule is in the stage of Felix's figure 561 b, consisting 

 of a Wolffian duct and a mass of nephrogenic epithelium. Here, instead of an 

 artery which can be injected, the section shows a chain of angioblasts running 

 ventral to the nephritic tissue to the lateral cardinal vein, and other sections show 

 similar chains of angioblasts connecting the aorta and the posterior cardinal vein. 



G ' "' Transverse section of an embryo pig of 23 somites, passing 

 through one of the lateral arteries of the pronephros. The section 

 is from a s P ec >men of the same litter as the one shown on plate 1, 



figure j and from the same serieg as figures 5 and 6 The lcvel of 



the sec t ioi >s shown by a line on plate I. figure 1. The section is 

 20 /i thick and is stained with hematoxylin and counterstained with 

 orange G, eosin, and aurantia. X115. A. om., a. omphalo-mesen- 

 terica; A. pr., a. of the pronephros which was injected from the 

 aorta ; r. m.,v.omphalo-mesenterica;K.c.i.,v.cardinalislateralis; 



v - c ' p " v - cardinalis posterior; w. *., woiman duct. 



