6 NATURAL HISTORY OF VERTEBRATES. 



wards, and the part of the intestine behind the anus, as well as the neurenteric canal, 

 soon become obliterated. It is at a late stage that the intestine acquires a new open- 

 ing to the exterior, the mouth (Fig. 5), which is formed by the turning in of the epi- 



V)last towards the front end of tlie intestine. 

 In this way a complete intestinal tube is 

 established. 



While these changes in the neural and 

 intestinal tubes are going on, important 

 alterations are likewise taking place in the 

 mesoblast. This layer is not continuous 

 underneath the neural tube, but is divided 

 into a right and left half (Fig. 4), separated 

 Ity the notochord, itself prob.'ibly derived 



Fig. 5.— Longitudinal section of Bombinator embryo; f,.„,„ the underlvilio- livl)obla';f It is imnor- 

 a/i, anus ; »ir,notocbor<l;/,liver; m, moutli; mc, neural ""'" i"^ unntujing ji^puui.iM. it is impor- 

 tant that the student should understand the 

 relationship of the mesoblast to the noto- 

 chord at this stage, as it is round the notochord that the important axial skeleton is 

 eventually formed. At present each sheet of mesoblast is plainly formed of two strata, 

 Fig. 3, the outer of which applies itself to the epiblast, the inner to the hypoblast. The 

 space between the two is the future cavity of the bodj^ in which the viscera lie; higher 

 up the space dilates considerably in the parts of the mesoblastic sheets lying alongside 

 the notochord, and eventually a complete separation is effected between these upper 

 and lower parts. The ujijier parts become divided into a number of cubical segments, 

 arranged in pairs on either side of the notochord ; these retain their original cavity for 

 some time, and give rise to certain very important organs. They are known as proto- 

 vertebras, or, better, as mesoblastic somites, and are the first indication tliat the frog 

 is a segmented animal. The lower parts of the mesoblast, or lateral ])lates as they 

 are now called, remain undivided into segments. 



All of the processes above described, as well as others to which no reference has 

 been made, take place while the embryo is still encased in the egg-membrane. After 

 that membrane has been l^urst, the embryo escapes and swims about as a tadpole, which 

 still differs from the adult by the presence of a fish-like tail, the absence of paired 

 limbs, and the fact that respiration is conducted by external gills instead of by lungs. 

 The change to the air-breathing adult stage (so great that it is called a metamor- 

 phosis) is effected by the loss of the tail and gills, and the development of the legs and 

 lungs. 



It is by processes similar to tliose described in the frog that the unicellular egg of 

 all vertebrates is converted into an embryo in which three distinct layers of cells are 

 present, destined to give rise to all the organs of the body. These layers are arranged 

 relatively to each other in such a way that the epiblast which covers the whole body 

 has already given rise to a dorsal nervous tube, sejjarated by the notochord from the 

 primitive intestine, while the mesoblast, situated between the other layers, has sepa- 

 rated dorsally into the metameric cubical somites, while ventrally it splits into two 

 plates, which adhere to the epiblast and hypoblast respectively, leaving between them 

 the body-cavity. 



These processes have only been described so far as to make intelligible the mode of 

 origin of the different systems of organs, to the account of the general arrangement 

 of which we now proceed. They are generally discussed in the following order : the 



