228 MANUAL OF ANIMAL BIOLOGY 



fuse in the mid-ventral line, thus forming a complete layer of meso- 

 derm, just under the ectoderm and enclosing the endoderm. 

 Before this process has been completed, each lateral plate splits 

 into an outer layer, known as the somatic layer, which is closely 

 applied to the ectoderm of the body wall, and an inner layer, the 

 splanchnic layer, which encloses the endoderm of the enteron, 

 and later gives rise to the supporting and muscular elements of 

 the wall of the alimentary canal. Between the somatic layer and 

 splanchnic layer a cavity develops which is the permanent body 

 cavity, or coelom. 1 



Rapid growth of the embryo continues, and in a few days after 

 fertilization a free-swimming, fish-like tadpole has developed with 

 definite head, trunk, and tail. By this time the embryo has 

 hatched ; that is, it has emerged from the capsule of jelly enclosing 

 it. The enteron has developed into a long, coiled intestine 

 which is typical of herbivorous animals. External respiratory or- 

 gans are present in the form of branched gills which arise from 

 each side of the head. These persist only a very short time and are 

 replaced by internal gills lying in the gill slits which open into 

 the pharynx. Definite sense organs (eye, nose, and ear) can be 

 identified in the head region. The tail has grown posteriorly to a 

 considerable length. Along each side of the body the primitive 

 muscle segments, myotomes, as noted, can be seen lying inside 

 the outer ectodermal covering. Limbs are not present at this 

 time. (W. f. 175.) 



4. Derivation of the Organs 



As already noted, all the tissues and organs of the Frog develop 

 from the three primary germ layers. The tissues derived from the 

 ectoderm include the epidermis which forms the outer covering 

 over the entire body, also the central nervous system with all of its 

 later development. 



The endoderm gives rise to the original wall of the primitive 

 enteron, and these cells remain as the permanent lining of the 

 alimentary canal so that, even in the adult condition, the alimen- 

 tary canal is lined for almost its entire length by cells which have 

 been derived from the endoderm. There is a portion of the mouth 

 cavity (stomodaeum) where the ectoderm is slightly invaginated, 



1 This development of the coelom is essentially the same as in the Earthworm 

 which is illustrated in W. f. 173, J, K. 



