202 PROFESSOR MARSHALL AND EDWARD J. BLES. 



unite to form a commissural vessel immediately behind the infundi- 

 bulum. The ventral branches, which are very slender, continue 

 forwards, and end blindly close to the median plane, between the 

 brain and the pharynx, and on a level with the eye-stalks. 



Posteriorly to the division into dorsal and ventral branches, the 

 aortse are large vessels, somewhat compressed dorso-ventrally. The 

 opening of the first branchial efferent vessel is wide, that of the second 

 somewhat narrower. 



Behind the pharynx the two aortse approach each other, and about 

 the level of the anterior nephrostomes they meet and unite to form 

 the single dorsal aorta. This may be traced backwards, diminishing 

 in size, as far as the posterior or third nephrostomes, beyond which 

 level it does not extend at this stage. 



Blood corpuscles occur in the dorsal aortse, but are very few in 

 number. About the level of the first nephrostomes the aortse show 

 slight bulgings (Fig. 4, G) on their ventral walls, which are the 

 commencements of the glomeruli. 



6. The Condition of the Blood Vessels at Stages Earlier 



THAN i\ MM. 



It will be convenient if we give here the results of our observations 

 on the condition of the blood vessels in tadpoles at stages earlier 

 than 4^ mm. 



a. The Blood Vessels in 4 mm. Tadpoles. — At this stage there are no 

 vessels in the visceral arches themselves. The dorsal aortse are just 

 commencing to form in the roof of the pharynx, each consisting at 

 present of three independent lacunar spaces opposite the dorsal ends 

 of the arches. Of these spaces, the first or most anterior one lies 

 opposite the posterior part of the hyoid arch ; the second or middle 

 one is the largest, and extends from opposite the middle of the first 

 branchial arch to the posterior part of the second branchial arch ; 

 the third space is small, and lies opposite the third branchial arch. 



b. The Blood Vessels in i\ mm. Tadpoles. — The dorsal aorta is now 

 a continuous vessel on each side, the three separate lacunse of the 

 earlier stage having opened out into one another. It now extends 

 from the level of the infundibulum to the posterior end of the 

 head. In the middle part of its course, opposite the hyoid and 

 first two branchial arches, the aorta is now acquiring definite walls ; 



