THE DEVELOPMENT OF VERTEBRATA. 



absorbed. The liver has branched repeatedly, and is be- 

 ginning to assume the compact adult character ; the pancreas 

 has appeared as a pair of diverticula behind, and quite in- 

 dependent of, the liver. The lungs, too, began to appear 

 very early as a pair of pits in the floor of the hindmost 

 part of the pharynx. 



Blood-vessels have been developing in all parts, and joining 

 up. The most important of these, at first, is the sub-intestinal- 

 vein, of which the heart is but the anterior part. This 

 serves to carry the absorbed yolk from the mesenteron to 

 the heart, and so to the body generally, before feeding 

 begins. As the liver develops two series of branches grow 

 out of this vein and form a portal system, more and more of 

 the blood passing this way, until the original direct com- 

 munication becomes closed, and the sub-intestinal vein 

 divided into portal and hepatic veins. The chief other 

 blood-vessels have already been mentioned in connexion 

 with the gill-slits and pronephros. A general diagram of 

 the circulation is given in fig. 124. 



Considerable changes have taken place in the nervous 

 system, and nerve-fibres have grown out into all parts of 

 the body, as must needs be the case before such active work 

 as eating and swimming can be carried on. An interesting 

 point in the tadpole is the development of a series of sensory 

 tubules along the lateral line, as in fishes. Considerable 

 histological differentiation has also taken place in all parts. 

 But consideration of these and other details would lead us 

 too far away from the main course of development. They 

 will be better considered at a later stage. 



17. Preparations for the Adult Stage. It is more 

 important to note the appearance of organs that are 

 useless to the tadpole, but needful to the frog, and which 

 now begin to develop wherever they can without interfering 

 with the efficiency of the larval organs themselves. The 

 lungs have been already mentioned. As they grow in size, 

 an artery branches oft* from the fourth efferent branchial 

 of either side to supply them. 



The fore limbs are developed under the opercular fold, 

 so as not to be in the tadpole's way ; but the hind ones 



