76 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



That there are vessels within the embryo at the stage of 12 somites which can 

 be injected from the aorta is proved by three sections from an injected chick of this 

 stage (text-figs. 1 to 3). These sections are best followed by comparing them with 

 the specimen shown in plate 2, figure 1, from a chick with 14 somites. The section 

 shown in text-figure 1 passes through the midbrain and shows a plexus of vessels 

 on the midbrain fully as large as the aorta itself. On the left side of the section 

 i right side of the embryo) is a slender artery containing ink, connecting this plexus 

 with the aorta. This plexus of large vessels on the midbrain, shown in text-figure 1, 

 also connects with a single longitudinal vessel along the hindbrain at this stage. 



These neural vessels, which at this stage are connected with the aorta, have 

 no vent, which probably explains the great difficulty in injecting them. They 

 are full of fluid, and though the ink enters them from the aorta, it does not penetrate 

 far (text-fig. 1). This point is, I think, interesting in connection with the time of 

 the beginning of circulation. As is well known, the heart begins to beat early in 

 the second day. I have made a number of observations which show that it beats 

 at the stage of 10 somites. In one instance I injected an embryo of 10 somites in 

 which the heart was not beating, and 

 when a small amount of the ink entered 

 the heart it was stimulated to beat. In 



another instance I had been watching 



ienlitprl VilietnrWm nf Q nmitps fnr FIG. 3. Transverse section of an injected chick of 12 somites 



passing through the twelfth interspace to show the rela- 



OVer an hour When the heart began tO tion of the Posterior cardinal veins to the aorta. .The 



section is from the same series as figures 1 and 2, and is 



beat. ThlS OCCUrred jUSt as the tenth to be compared with the total preparations shown on 



. . i TJ plate 1, figure 2, and on plate 2, figure 1. The section is 



SOmite WaS beginning tO appear. It IS 3 M thick and is unstained. xm. AO., aorta; \. 



therefore quite certain that the stage of ue P hrotome : v - cardinal* posterior. 



10 somites marks the beginning of the heart-beat. 



At the time the heart begins to beat its venous end connects with the exten- 

 sive capillary plexus of the area pellucida in which the omphalo-mesenteric veins 

 arise, and the entire aorta opposite the myotomes is connected with the capillary 

 plexus in which the omphalo-mesenteric arteries arise. In other words, there is 

 a plexus of vessels covering the entire area opaqua and area pellucida which con- 

 nects with the venous end of the heart and with the entire dorsal aorta of the 

 embryo opposite the zone of the myotomes. In the area pellucida this plexus 

 of vessels is filled with fluid, but there are very few free cells in the vessels. After 

 the heart begins to beat most of the isolated blastoderms show occasional wander- 

 ing cells of various types that float into the vessels of the area pellucida, showing 

 that these vessels are full of fluid; and when one of these cells approaches the 

 heart in the omphalo-mesenteric veins it oscillates back and forth with each beat. 

 It is thus very strikingly apparent that the circulation does not begin for a con- 

 siderable time after the heart begins to beat. It is difficult to note the exact 

 time of the beginning of the circulation while the chick is on the yolk, for the 

 few red blood-corpuscles that are forced into the aorta are inconspicuous with 

 the powers of the microscope that can be used. In the isolated blastoderms the 

 earliest chick in which I have seen the circulation begin was one of 17 somites. 

 At the beginning of circulation a few corpuscles are shot into the aorta with each 



