282 



ZOOLOGY 



SECT. 



the central nervous system. Posteriorly they are continuous 

 with the lips of the blastopore, so that when the neural groove 

 becomes closed in behind, the archenteron, as in Amphioxus, 

 communicates with the neuroccele by a neurenteric canal (K, n. e. c.). 

 The embryo soon begins to elongate ; one end is broad, and, 

 becoming separated by a slight constriction, is marked out as the 

 head : the other end is bluntly pointed and is the rudiment of 

 the tail (t.). On the ventral surface of the root of the tail a procto- 

 dceum (pcdm.) appears and communicates with the archenteron. 



The head and 

 tail become 

 more distinctly 

 marked off from 

 the trunk. A 

 pit the stomo- 

 dceum (J L, st. 

 dm.) appears 

 on the antero- 

 ventral surface of 

 the head, and, 

 immediately be- 

 hind it, a semi- 

 lunar area with 

 raised edges, the 

 sucker (sk.). At 

 each side of 

 the head two 

 branched pro- 

 cesses appear: 

 they are the ex- 

 ternal gills (br.l., 

 br.%.\ and the 

 regions from 

 which they arise 

 mark the posi- 

 tions of the first 

 and second bran 

 chial arches. 



The embryos are now hatched as tadpoles. They swim freely 

 in the water or adhere to weeds by means of their suckers (Fig. 

 951, 7). They are still blind and mouthless, the stomoda3um 

 not having yet communicated with the archenteron. Soon a third 

 pair of external gills appears on the third branchial arch, and the 

 first two pairs increase greatly in size (2, 2a) : the stomoda3um joins 

 the archenteron, gill-slits (branchial clefts) are formed between the 

 branchial arches, and the eyes appear. The mouth is small, 

 bounded by lips beset with horny papillae and provided with a 



FIG. 951. Rana temp or aria. Stages in the life-history, 

 from the newly-hatched Tadpoles (1) to the young Frog (8). 

 2a is a magnified view of 2. (From Mivart.) 



