EMBRYOLOGY 495 



operculum, the gill-clefts close, and finally the tail shortens 

 and is absorbed, and the metamorphosis is complete. 



We have traced the internal development of the embryo 

 „ up to the establishment of the three layers of 



the body of a triploblastic animal. Only an 

 outline of the further development of these layers can be 

 given. From the epiblast or embryonic ectoderm arise 

 the epidermis, nervous system, sense organs, and lining of 

 the mouth and cloacal opening; from the hypoblast or 

 embryonic endoderm arises the lining of the greater part 

 of the gut, the lungs, liver, pancreas, and thyroid, and the 

 notochord; from the mesoblast or embryonic mesoderm 



e.g. au.c. 







Fig. 375. — A frog embryo at the stage of hatching. 



««., Proctodeum ; au.c, slight swelling over the rudiment of the ear ; e.g-., external 

 gills on gill arches ; na., invagination to form nasal capsule ; o.c, slight swelling 

 over the rudiment of the eye ; s., sucker ; stm., stomodeeum (invagination 

 which will form the mouth). 



arise the skeleton, connective tissues, muscles, vascular 

 system, excretory organs, and generative organs. The 

 skeletal tissues and unstriped muscle arise from a loose 

 kind of mesoderm, known as mesenchyme, formed of cells 

 budded off by the compact mass around the coelom and 

 perhaps also by the ectoderm and endoderm. The mass 

 around the coelom is known as mesothelium, and from it 

 arise all the remaining mesodermal tissues. 



The origin of the central nervous system has already been 



described. The dorsal roots of the nerves are 

 Nervous formed as growths from the edges of the neural 



lenseTorgans. plate before the neural folds have met. The 



ventral roots arise later, as outgrowths from the 

 side of the central nervous system, and those of the spinal 



