DEVELOPMENT OP THE BLOOD VESSELS IN THE PROG. 263 



portions between the arterial and venous systems of the 

 tadpole and those of an adult Elasmobranch, such as a 

 dogfish. 



34. The external and internal gills apparently form one con- 



tinuous series of structures. 



35. The lungs appear very early, and are used for a considerable 



period simultaneously with the gills. 



36. The oesophagus becomes solid for a period commencing 



shortly before the mouth opens, and lasting some little 

 time after the opening is effected. 



37. The tadpole undergoes a distinct increase in bulk prior to the 



opening of the mouth. We have suggested that in the 

 early stages nutriment is absorbed through the sucker. 



38. There appear to be amongst Amphibians great differences 



between families, genera, species, and even individuals in 

 the actual mode and relative times of development of the 

 blood vessels of the mandibular, hyoidean, and branchial 

 arches. Our present information is far too imperfect to 

 enable us to determine which of these modes is to be 

 regarded as typical for the group. 



