INTRODUCTION 519 



sharks, mammals) or within well-protected egg membranes (turtle, chick) 

 slur over the larval condition. 



The larval stage in non-viviparous fishes (see Kyle, '26, pp. 74-82) and in 

 the majority of amphibia is a highly differentiated condition in which the 

 organs of the body are adapted to a free-living, watery existence. The tadpole 

 of the frog, Rana pipiens, from the 6-mm. stage to the 11 -mm. stage, presents 

 a period during which the primitive embryonic condition, present at the time 

 of hatching (i.e., about 5 mm.), is transformed into a well-developed larval 

 stage capable of coping with the external environment. From this time on to 

 metamorphosis, the little tadpole possesses free-living larval features. Another 

 example of a well-developed, free-living, larval stage among vertebrates is 

 that of the eel, Anguilla rostrata. Spawning occurs in the ocean depths around 

 the West Indies and Bermuda. Following the early embryonic stage in which 

 primitive body form is attained, the young transforms into a form very unlike 

 the adult. This form is called the Leptocephalus. The Leptocephalus was for- 

 merly classified as a distinct species of pelagic fishes. After many months in 

 the larval stage, it transforms into the adult form of the eel. The latter migrates 

 into fresh-water streams, the American eel into streams east of the Rockies 

 and the European eel into the European streams (Kyle, '26, pp. 54-58). The 

 larval stages in most fishes conform more nearly to the adult form of the fish. 



The embryo of Squalus acanthias at 20 to 35 mm. in length, the chick 

 embryo at 5 to 8 days of incubation, the pig embryo of 12- to 18-mm. length, 

 and the human embryo of 12 to 20-mm. length may be regarded as being 

 in the stage of larval transition. The young opossum, when it is born, is in a 

 late larval state. It gradually metamorphoses into the adult body form within 

 the marsupium of the mother (Chap. 22). 



3) Definitive Body Form. The general form and appearance of the adult 

 constitute definitive body form. The young embryo of Squalus acanthias, at 

 about 40 mm. in length, assumes the general appearance of the adult shark; 

 the frog young, after metamorphosis, resembles the adult frog (Chap. 21), 

 the chick of 8 to 13 days of incubation begins to simulate the form of the 

 adult bird; the pig embryo of 20 to 35 mm. gradually takes on the body fea- 

 tures of a pig, and the human fetus, during the third month of pregnancy, 

 assumes the appearance of a human being. The transformation of the larval 

 form into the body form of the adult is discussed further in Chapter 21 in 

 relation to the endocrine system. 



3. Basic or Fundamental Tissues 



Through the stages of development to the period when the primitive or 

 generalized, embryonic body form is attained, most of the cells which take 

 part in development are closely associated. In the primitive embryonic body, 

 this condition is found in all the five primitive body tubes and in the noto- 

 chord. These closely arranged cells form the primitive epithelium. In the de- 



