120 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. 



sexual animal. The development of larvje, however, is by no means 

 direct and uniform, but is complicated by the necessity for special 

 contrivances to enable them to procure food and to protect them- 

 selves ; sometimes taking place in an entirely different medium, 

 under different conditions of life. This kind of post-embryonic 

 development is known as metamorphosis. 



Well-known examples of metamorphosis are afforded by the deve- 

 lopmental histories of the Insecta and Amphibia. From the eggs 

 of Frogs and Toads proceed larvse provided with tails, but without 

 limbs, the so-called Tadpoles (fig 111). These, with their laterally 

 compressed tails and their gills, remind one of fishes, and they possess 

 organs of attachment in the form of two small cervical suckers by 

 which they can anchor themselves to plants. The mouth is provided 

 with horny plates ; the spirally coiled intestine is surprisingly long ; 

 the heart is simple; and the vascular arches have the piscine relations. 

 Later, as development proceeds, the external branchiae abort, and are 

 replaced by new branchiae covered by folds of the integument, the 

 caudal fin is enlarged, and the fore and hind limbs sprout out ; the 

 fore limbs remain for some time covered by the integument, and only 

 subsequently break through it to appear on the surface. Meanwhile 

 the lungs have developed as appendages of the anterior part of the 

 alimentary canal, and supplant the gills as respiratory organs, a 

 double circulation is developed, and the horny beak is cast off. 

 Finally the tail gradually shrinks and atrophies ; on the completion 

 of which the metamorphosis of the aquatic tadpole into the frog or 

 toad suited for life on land is accomplished (fig. 112). 



We have then to consider two kinds of development, viz., develop- 

 ment with a metamorphosis and direct development, which in extreme 

 cases are distinctly opposed to each other, but are connected by inter- 

 mediate methods. The size of the egg, or, in other words, the amount 

 of food yolk available for the use of the embryo in proportion to the 

 size of the adult animal appears to be a factor of primary importance 

 in any explanation of these two distinct processes (R. Leuckart). 

 Animals with a direct development require generally in pro- 

 portion to the height of their organization and the size of their 

 bodies that their eggs should be provided with a rich endowment 

 of food yolk, or that the developing embryo should possess a special 

 accessory source of nutriment ; they arise therefore either from 

 relatively large eggs (Birds), or they are developed inside, and in 

 close connection with the maternal body, by which arrangement 

 they have a continual supply of food material (Mammals). Animals, 



