Emhryologic Development of Animals 447 



(medullary plate) in the ectoderm (Fig. 222). During this time, the 

 embryo grows longer and has definite anterior and posterior ends. A 

 thickened fold at each margin of the original neural plate forms a neural 

 fold (medullary fold). These folds at first are flat and far apart. Later 

 they arch toward the median dorsal line and unite to form the future 

 neural tube. At this time the neural plate sinks to form a definite neural 

 groove along the middorsal side of the embryo. The neural groove is 

 composed of ectoderm cells (Fig. 222) . 



An elongated mass of cells dorsal to the archenteron forms the long, 

 rodlike notochord which may still be connected with the entoderm from 

 which it originates. The mass of cells at either side of the neural groove 

 is known as the mesoderm (middle germ layer) (Fig. 222) . 



The neural tube stage closely follows the neural groove stage (Fig. 

 220, /) . The two neural folds on the dorsal surface of the embryo at 

 this time have met and fused into an elongated neural tube (Fig. 222). 

 At this stage the latter probably will be free from the outer ectoderm 

 from which it originated. The anterior part of the neural tube constricts 

 and enlarges by well-regulated mitosis to form the future fore-, mid- and 

 hindbrains. The notochord is now free from the archenteron and is just 

 below the neural tube (Fig. 222) . 



The mesoderm completely surrounds the archenteron ventrally, arid 

 near the middorsal part there appears a small split or break which is the 

 forerunner of the coelom (body cavity). This break continues ventrally, 

 thus forming the body cavity between the two layers of the mesoderm. 

 The inner layer of the mesoderm, known as the splanchnic layer, lies 

 next to the entoderm, while the outer layer, known as the somatic layer, 

 lies next to the ectoderm. The cells of both ectoderm and entoderm are 

 now quite distinct (Fig. 222). 



The larval stage with external gills follows the neural tube stage. A 

 pair of oval, thick-lipped suckers on the ventral side of the tadpole serve 

 for attachment purposes. The stomodeum (primitive mouth) appears as 

 an oval pit in front of the suckers. The olfactory pits are a pair of small 

 depressions above and anterior to the stomodeum. The three pairs of 

 external gills are fingerlike processes on either side of the head which act 

 as specialized organs of respiration. The proctodeum (primitive anus) 

 is located on the dorsoposterior part of the tadpole. A tail and a pair 

 of eyes are also present (Fig. 221 ) . 



The larval stage with internal gills (Fig. 221) follows the stage with 

 external gills. The external gills are now covered by a fold of skin known 

 as the operculum (gill cover) which has a single opening called the 



