376 THE NEW INDIVIDUAL Part IV 



in which the gills are functioning. Then, with the approach of metamorphosis, 

 the lung sacs enlarge but the endoderm at their tops is constricted in prepara- 

 tion for the future larynx. Although the tadpole is still a true water breather, 

 it is also a presumptive air breather. Before the gills are spent, the lungs are 

 ready to begin work. For the gills and the lungs it is a case of: "The king is 

 dead! Long live the king." 



Mesoderm — The Bulk of the Body. The mesoderm produces the connective 

 tissue, the skeleton, the blood and blood vessels, the muscles and other parts 

 including the lining of the body cavity, the kidneys and the reproductive sys- 

 tem. 



Metamorphosis from Tadpole to Frog 



During the change to adult form in the larvae of frogs and toads, the tail 

 and gills are absorbed; the gill clefts are closed; legs develop; lungs become 

 functional; and the food cavity is changed. The horny lips with which the 

 tadpole scrapes algae are replaced by bony jaws and teeth; the relatively long 

 "watch spring" intestine is changed into a shorter one that functions with a 

 mixed diet of plants and animals (Fig. 34.7). 



Provisions for Health and Safety of Embryos 



Developing embryos are provided with water and food. They use water 

 continually and it forms a large part of their substance. They have prospered 

 in watery surroundings throughout their histories. The delicate embryos of 

 aquatic animals float and swim in lakes and seas. The equally delicate embryos 

 of most land animals develop within sacs of fluid, individual ponds that take 

 the place of the wider waters of their aquatic relatives. 



Earthworms pass their early days within seed-like capsules. Each of these 

 holds a few embryos in a bath of nourishing albumen which they swallow and 

 also absorb through their skins. Like those of other invertebrates, these em- 

 bryos have no special food-sacs attached to their bodies. 



Food and the Yolk Sac. The majority of vertebrates, fishes, reptiles, birds, 

 and mammals, have a yolk sac containing more or less food in the form of 

 yolk. It is a pouch-like extension of the digestive tract, an organ producing 

 enzymes that break the yolk into substances that pass into the blood, are car- 

 ried into the body of the embryo, and finally converted into its protoplasm. 

 In birds, the body wall closes over the yolk sac before hatching and the latter 

 shrinks and finally merges into the intestine. The rounded front of a one-day 

 chick is due to its yolk sac. 



The Watery Environment and the Amniotic Sac. The amnion is a trans- 

 parent roomy sac that loosely surrounds the embryo (Figs. 19.13, 19.14). It 

 contains the amniotic fluid secreted by the membranous sac and by the embryo 

 itself. The fluid allows the embryo considerable free motion especially during 



