230 



ESSENTIALS OF ZOOLOGY 



is resolved into the paired myotomee above and the splanchnic 

 mesoderm below. Both ultimately develop cavities, forming 

 the segmented myocoels and the splanchnocoel. Head and 

 mandibular cavities are not formed as such, and the eye 

 muscles appear to be developed from solid groups of mesen- 

 chyme. 



When the larva escapes from the egg it is in an incom- 

 plete state of development. The mouth and the gills can be 

 seen as grooves externally, but they are not perforated ; the 

 cloaca is the only opening. Behind the region of the mouth 



Fore 



Proneph. 

 ch . Entercn duct Postanal ffut 



'Cloaca 



Stomod. 



Mesenchyme 



FIG. 118. Diagrammatic longitudinal section of tadpole at hatching. 

 After Marshall. 



a horseshoe-shaped pit is formed which is glandular and acts 

 as an adhesive organ or cement gland. With this the larva 

 attaches itself to the albuminous mass from which it has 

 escaped, and afterwards to plants. The cement gland prevents 

 the larva falling into the mud at the bottom of the pond. 

 The ridges between the gill grooves develop tuberances, at 

 first two pairs and then three pairs, which grow out into 

 branching, vascular, external gills. Many of the ectodermal 

 cells of the body are ciliated. The larva continues to grow, 

 using up the yolk contained in the cells of the floor of the 

 enteron. The tail gains in length as a laterally flattened 

 structure, provided above and below with median membranous 

 fins and containing an extension of the notochord and myo- 

 tomes, which, as in fishes, are < -shaped. As it lengthens, the 

 postanal endoderm degenerates into a rod of cells and 



