DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 97 



THE VASCULAR SYSTEM IN YOUNG PIG EMBRYOS. 



In the study of the vascular system in a mammal it is not as easy to obtain 

 young stages for injections, as in the case of the chick. The material, however, 

 offers valuable opportunities for comparison with human embryos, and to obtain 

 injections in much earlier stages than have ever been injected in human specimens. 

 I shall follow the development of the vessels in the pig by the aid of six figures of 

 injected embryos, and shall describe the specimens and follow the development 

 of the vessels under six headings: First, the form of the heart; second, the ventral 

 branches of the aorta, including the allantoic arteries and the subintestinal artery; 

 third, the umbilical veins and the vessels of the thoracic body-wall; fourth, the 

 vascular system of the nervous system and the formation of the primary head-vein; 

 fifth, the cardinal veins; sixth, the vessels of the pronephros and the mesonephros. 



THE FORM OF THE HEART. 



The youngest pig which I have injected is shown on plate 4, figure 3. This 

 is from a specimen which measures 4 mm. in oil and which has 14 somites. It 

 corresponds in development with a human embryo, Xo. 470 of the Carnegie 

 collection, which measures 3.3 mm. and is in the fourth week of development. 

 In this embryo pig an injection was made into the aorta opposite the origin of the 

 omphalo-mesenteric arteries. The point of injection was obscured by extravasa- 

 tion, so that it is not shown in the drawing. The stage of development of the 

 specimen can be judged by the form of the brain, the otic vesicle, and the form 

 of the heart. 



The extensive venous plexus covering the anterior or cephalic wall of the 

 yolk-sac converges on either side into large right and left omphalo-mesenteric 

 veins, which meet in a conjoined tube, the sinus venosus. The sinusoids of the 

 liver have not yet begun to form, so that the sinus venosus stands out clearly. 

 The sinus has a marked diverticulum, which Tandler called the horn. The dorsal 

 wall of the sinus shows a series of sprouts, representing the duct of Cuvier, which 

 is probably developed at this stage, as indicated by the posterior cardinal vein, 

 but is incompletely injected. The most caudal of the sprouts form a small plexus 

 representing the umbilical vein in the somatopleure. 



Above the sinus venosus is a well-marked groove between the sinus and the 

 atrium. The atrio-ventricular canal, on the other hand, is only just indicated. 

 The form of the heart corresponds closely with the description by Tandler (Manual 

 of Human Embryology, Keibel and Mall, page 536), which is based on the studies 

 of Born, in which he says that the heart becomes a horizontal loop, the two limbs 

 of which are separated by an almost horizontal bulbo-ventricular cleft into two 

 parts, a ventricular limb and a bulbar limb. In my specimen the bulbar limb 

 consists of three parts: first, the bulbus cordis; second, a short constricted portion 

 of the tube, the f return Halleri; third, the large truncus arteriosis, which gives 

 off the two aortse. In the use of the term /return Halleri I am following the usage 

 of His (Anatomic menschlichen Embryonen, pages 131 and 140). He describes this 

 portion of the tube as the portion which ultimately gives rise to the semilunar valves. 



