222 PROFESSOR MARSHALL AND EDWARD J. BLES. 



The Cuvierian veins are larger than before, and now considerably 

 exceed the hepatic veins in size. Of their two components, the 

 posterior cardinal veins or sinuses are greatly dilated in relation with 

 the head kidneys ; while the anterior cardinals or jugular veins are 

 also of large size, extending forwards along the sides of the head 

 about the level of the notochord (Fig. 10, V R). 



The great system of lacunar venous sinuses on the ventral 

 surface of the head, in front of the pericardium and above the sucker, 

 has already been alluded to; and the mandibular and hyoidean veins 

 opening into these have been described (Fig. 7). From these large 

 sinuses the blood is returned to the heart by the inferior jugular 

 veins, a pair of large vessels which run backwards along the side 

 walls of the pericardium, and open into the Cuvierian veins just 

 before these join the sinus venosus. 



In many respects the arrangement and proportions of the veins of 

 the tadpole at this stage bear a striking resemblance to those of an 

 adult Elasmobranch, such as a dogfish. In this connection we may 

 note more especially the position and relations of the mandibular 

 vein, and its communication with the heart through the inferior 

 jugular vein, and also the enormous size of the front part of the 

 posterior cardinal sinuses. The arrangement and proportions of the 

 arteries of the head also support this comparison strongly. 



5. General Considerations. 



The most important points of a general nature that call for 

 consideration at this stage are connected with the vessels of the 

 mandibular and hyoidean arches. These can be only partially dealt 

 with at present, and we propose to defer their discussion until we 

 have described tadpoles of the next two stages — 9 mm. and 12 mm. 

 respectively. 



The exceedingly early period at which the pulmonary artery 

 developes is rather surprising, and deserves attention. 



V. THE CONDITION OF THE HEART AND BLOOD VESSELS 

 IN 9 mm. TADPOLES. 



1. External Characters. 



This is an interesting and well-marked stage, as it is at this period 

 that the mouth-opening is first formed, by perforation of the septum 



