116 



DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



A.^es 



FIG. 5. Transverse section of an embryo pig of 23 somites, passing 

 through one of the lower myotomes and the mesonephros, to show 

 the position of the subintestinal artery and a chain of angioblasts 

 which will form an artery of the mesonephros. The section is from 

 a specimen of the same litter as the one shown on plate 1, figure 1, 

 and from the same series as figures 4 and 6. The level of the section 

 is shown by a line on plate 1, figure 1. The section is 20 ju thick and 

 is stained with hematoxylin and counterstained with orange G, 

 eosin, and aurantia. X115. A. mcs., a chain of angioblasts 

 which connect the aorta with the v. cardinalis lateralis and which 

 will form an artery of the mesencephalon but were not injected 

 because they are still solid; A. si., a. subintestinalis; V. c. L, v. car- 

 dinalis lateralis; V. c. p., v. cardinalis posterior. 



These angioblasts are mesial to the nephritic tissue. This section is, I think, 

 similar to the section in Evans's figure 416 from a human embryo of the same 



stage, namely, with 23 somites, 

 which shows the posterior cardinal 

 vein dorsal to the Wolffian duct 

 and the lateral cardinal vein ven- 

 tral to the duct. 



A comparison of text-figures 4 

 and 5 seems to me to indicate that 

 the primary arteries of the nephro- 

 genic tissue are ventral to the ne- 

 phrotome, but when the tubules 

 are farther developed the artery 

 crosses the neck of the tubule; in 

 other words, the tubules grow ven- 

 tral to the arteries. 



The study of the embryo pig at 

 the stage of 23 somites (as shown in 

 plate 1, figure 1, and in the sections 

 of text-figures 4 and 5), seems to me to indicate that the posterior and lateral 

 cardinal vessels extend caudalward in connection with chains of angioblasts from 

 the aorta which pass dorsal and ventral to the nephritic tubules in lines which are 

 very plain in figure 5. 



In figure 5 of plate 3 is shown 

 a ventral view of the pronephritic 

 vessels in a pig of 20 somites, in 

 which it is clear that there is a tend- 

 ency toward a grouping of the 

 transverse arteries of the prone- 

 phritic tubules around segmental 

 lateral arteries. For example, be- 

 tween the ninth and tenth spinal 

 arteries there is one lateral artery 

 giving off four branches; between 

 the tenth and eleventh spinal ar- 

 teries are two lateral arteries from 

 the aorta, with three transverse 

 branches. 



The longitudinal artery shown 

 in plate 3, figure 5, persists for some 

 time in the pig and connects the glomerular arteries even after the arterial tufts 

 of the glomeruli are well formed. As seen in plate 3, figure 5, the transverse arteries 

 lead directly to a lateral vein, which in turn connects with the posterior cardinal vein. 

 Moreover, as is shown opposite the tenth somite, the posterior cardinal vein has 

 many direct connections with the aorta. 



FIG. 6. Transverse section of an embryo pig of 23 somites, passing 

 through the allantoic arteries to show that the primitive allantoic 

 arteries are in the splanchnopleure. The section is from a specimen 

 of the same litter as the one shown on plate 1, figure 1, and from the 

 same series as figures 4 and 5. The level of the figure is shown by a 

 line on plate 1, figure 1. The section ia 20 it thick and is stained with 

 hematoxylin and counterstained with orange G, eosin, and aurantia. 

 X53. A. al, artery of the allantois; C., ccelom; V. al., vein of the 

 allantois in the zone where the splanchnopleure, the somatopleure, 

 and the amnion are fused. 



