638 embryology: frog 



into toes. The fore-limbs arise at the same time as the hind- 

 hmbs, but as they are covered by the opercula they are not seen 

 till a later stage. About the end of the second month the lungs 

 which have been forming come into use and the gills start to 

 degenerate, and a fortnight later the tadpole begins to turn into 

 a young frog. The outer layer of the skin and the homy jaws 



are thrown off, the mouth en- 

 larges and changes its shape, the 

 fore-limbs appear, that on the 

 left being pushed through the 

 gill opening, that on the right 

 breaking through the operculum, 

 the gill-clefts close, and finally 

 the tail shortens and is absorbed, 

 and the metamorphosis is com- 

 plete. 



{"■'•■•"■'.'•'■Vl\ N 

 Xi,:--. 1-1 H SOT). 



GERM LAYERS 



We have traced the internal 

 development of the embryo up 

 to the establishment of the three 

 layers of the body of a triplo- 

 blastic animal. From the em- 

 bryonic ectoderm arise the epi- 

 dermis, nervous system, sense 

 organs, and lining of the mouth 

 and cloacal opening ; from the 

 embryonic endoderm arises the 

 lining of the greater part of the 

 gut, the lungs, liver, pancreas, and thyroid ; from the embryonic 

 mesoderm arise the skeleton, connective tissues, vascular system, 

 muscles, excretory organs, and generative organs. The skeletal 

 tissues and unstriped muscle arise from a loose kind of meso- 

 derm, known as mesenchyme, formed mainly of cells which 

 break away from the compact layer around the coelom (chiefly 

 by a break-up of the sclerotome), but also of cells which migrate 

 from the ectoderm and endoderm. The mass around the coelom 

 is known as mesothelium, and from it arise all the remaining 

 mesodermal tissues. 



Ir..- 



FiG. 498. — A diagram of a transverse 

 section of the frog embryo at the 

 hatching stage. 



ccel., Coelom ; ect., ectoderm ; gl., glomus, 

 receiving a branch from one of the two 

 suprabranchial arteries which a little further 

 back join to form the dorsal aorta (cf. Fig. 

 474 A) ; int., intestine ; Ir., liver ; m.pl., 

 muscle plate ; nch., notochord ; nst., nephro- 

 coelomostome ; s.d., segmental duct ; sop., 

 somatopleure ; sp.c, spinal cord. 



