414 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



At a later period, the nasal pits arise from the sensory plate as 

 ectodermal invaginations antero-ventral to each optic vesicle. 

 The beginnings of the gill clefts may now be seen upon the gill plate 

 (Fig. 215 D) as vertical grooves separating the branchial arches, or 

 gill bars; while the muscle segments become evident externally as 

 a series of < shaped markings. At the posterior end is the 

 proctodceum, or invagination of ectoderm that forms the anus 

 (c/. Fig. 215 E). It will be recalled that this lies at a point that 

 was once the ventral lip of the blastopore (cf. Fig. 213 I). The 

 outer surface of the embryo has ciliated areas at this stage, and 

 it rotates within its capsule. 



In later stages the external shape of the tadpole becomes 

 increasingly apparent by rapid outgrowth of the tail and develop- 

 ment of the caudal fin, until a length of six to seven milHmeters 

 is reached at an age of one or two weeks, depending upon the 

 temperature. Rudiments of what are called the external gills 

 appear shortly before hatcliing as outgrowths upon the gill plates 

 (cf. Fig. 219). Up to this point the developing individual may be 

 referred to as an "embryo," because it is enclosed within the 

 jelly in the way that many other animals are enclosed in egg 

 shells or other protecting membranes. It may, however, be 

 called a "tadpole" as soon as it has assumed these characteristic 

 features. By the same terminology, the tadpole is a "larva," and 

 the "larval period" begins when the "embryonic period" ends, 

 at the time of hatching. In hatching, the tadpole of the leopard 

 frog penetrates the jelly head foremost, and squeezes through a 

 small opening to become free even before it is able to wriggle about 

 actively as in later stages. The ciha are probably effective in this 

 process. It is then seen chnging by its suckers (cf. Fig. 11 F, p. 

 23) to the remains of the jelly or to other objects, moving when 

 disturbed, but for the most part quiescent until it becomes more 

 tadpole-like in appearance. During this period the external gills 

 develop rapidly as branched structures and the stomodseum 

 breaks through into the anterior end of the enteron. The yolk 

 persists for a considerable period after the tadpole has begun to 

 feed upon the organic ooze covering submerged objects and the 

 active life has commenced. 



The external gills function as temporary respiratory organs, 

 before the internal gills, which are comparable with those of a 

 fish, develop. A fold that arises anterior to the gill region and 



