Ch. xtv] orgaxs from the mesoderm 153 



veins that have come down from the dorso-hiteral region of the 

 embryo. These are the Cuvierian veins formed on each side 

 by the union of the posterior and anterior cardinal veins. The 

 posterior cardinals bring back the blood from the head-kidneys. 

 Around tlie head-kidneys these veins form sinuses that are 

 enormously large. Each posterior cardinal also receives so- 

 matic veins from the posterior part of the body-wall. The 

 anterior cardinal veins bring back blood from the dorsal part 

 of the head-region. 



In a larva 4.1- mm. in length, the blood-vessels of the branchial 

 region have also appeared. The anterior end of the heart, the 

 truncus arteriosus, divides into a right and left branch, which 

 pass forward and laterally toward the base of the gill-region. In 

 the mandibular arch no vessels are as yet present. In the hyoid 

 arch an irregular space appears in the mesoderm. In the first 

 Ijraiieliial arches two vessels appear, a large efferent vessel (Fig. 

 46, for an older embryo) connected with the dorsal aorta, and a 

 smaller afferent vessel. The latter is at present without con- 

 nection. In the second branchial arch the conditions are like 

 those in the first. In the third branchial arch only a small 

 efferent vessel has as yet appeared. Xo vessels are present 

 at this time in the fourth branchial arch. The dorsal aorta is 

 represented by a paired vessel in the dorso-pharyngeal region. 

 Opposite the hyoid arch each branch of the dorsal aorta di- 

 vides into a dorsal and into a ventral branch. The dorsal 

 branches meet each other behind the infundibulum, Avhile the 

 ventral branch passes forward to end blindly (Fig. 46). The 

 two aort?e unite posteriorly into a single vessel at the level of 

 the pronephros (Fig. 46, A). 



The condition of the blood-vessels shortly after the tadpole 

 has left its envelopes (it is then 7 mm. in length) is illustrated 

 in Figs. 46 and 47. The heart has enlarged and is further 

 twisted on itself. The aortic bulb-portion and the auricuhir 

 and ventricular portions are distinctly' marked from each other 

 by constrictions of the tube. The right and left branches of 

 the aortic bulb have grown toward the gill-arches, and the 

 afferent vessels of the first and second branchial arches have 

 united with the ventral aortic branches AF^ and AF^. The 

 efferent branches, EF^ and EF^, of the first and second bran- 



