VERTEBRATE DEVELOPMENT 231 



is like the adult, except that it is smaller. It feeds and gradually 

 attains the sexually-mature, adult condition. (W. f. 175, L.) 



6. Life Processes in the Frog Embryo 



When an egg is fertilized by a sperm, it is then an embryo of a 

 new generation ; in other words, an independent organism, and as 

 such it must carry on the essential life processes if it is to survive. 

 It will be of value briefly to indicate how the Frog embryo is able 

 to do this. In the first place, the question of nutrition must be 

 considered. The process of cell division and all the other devel- 

 opmental phenomena require energy. This energy in the early 

 stages, and until the time when the alimentary tract is completed 

 by the mouth and anal openings, is obtained by utilizing the yolk 

 material present in the vegetal pole, which is supplied to all the 

 cells. 



From the one-celled condition until sometime after the tail-bud 

 stage, the embryo does not possess a vascular system. Inasmuch 

 as food material is scattered among the cells there is no general 

 problem with regard to the transportation of food. The greater 

 portion, however, is concentrated near the middle of the embryo, 

 ventral to the enteron and posterior to the pharynx. Apparently 

 excess food in this region is passed on to the other cells of the 

 embryo just as in Hydra and other animals where no vascular 

 system ever develops. 



Soon after the embryo has hatched from the egg jelly it begins to 

 swim vigorously, and later when the alimentary tract is completed, 

 the animal begins to ingest food. It is adapted for plant feeding, 

 but the tadpole will eat animal tissue when available. It is aided 

 in feeding by a special organ, the ventral sucker, just posterior 

 to the mouth. This enables the tadpole to remain attached to a 

 plant in the water and to rasp the plant tissues with horny projec- 

 tions on the jaws. 



Respiration must, of course, be continuous from the earliest 

 stages. At first this respiratory interchange takes place at the sur- 

 face of the embryo, through the jelly capsule. This method does 

 not long suffice, and coincident with the establishment of the 

 vascular system, gills are formed through which the interchange 

 of gases takes place. External gills are present for a few days. 

 Later they are covered by a fold of ectoderm (operculum), and 

 internal gills develop in the walls of the gill slits. In this condi- 



