518 BASIC FEATURES OF VERTEBRATE MORPHOGENESIS 



Chapters 6, 7, 9 and 10, produce several specific organs. Organogenesis is 

 concerned with the formation of these specific organs. A group of organs 

 which are associated together to execute one general function form an organ 

 system. The digestive system, for example, has for its general function that 

 of obtaining nourishment for the body. It is composed of a series of organs 

 integrated toward this end. The nervous system, similarly, is an assemblage 

 of specific organs devoted to the discharge of nervous functions. So it is with 

 the other systems of the organism. System development is concerned with 

 the genesis of such systems. The association of various systems, integrated 

 together for the maintenance of the body within a particular habitat, consti- 

 tutes the organism. Finally, the organism acquires a particular body form 

 because of the form, structure, and activities assumed by its organ systems 

 as a result of their adaptation to the functional necessities of the particular 

 habitat in which the organism lives. It should be urged further that this nice 

 relationship between form and structure, on the one hand, and functional 

 requirements, on the other, is a fundamental principle of development from 

 the egg to the adult. It is a principle intimately associated with the morpho- 

 genesis of the organ systems described in Chapters 12 to 21. 



During development from the egg to the adult form, three major types of 

 body form are evolved in the majority of vertebrate species. 



b. Primitive, Larval, and Definitive Body Forms (see fig. 255) 



1) Primitive Body Form. The condition of primitive or generalized, em- 

 bryonic body form is attained when the embryo reaches a state in which its 

 developing organ systems resemble the respective developing organ systems 

 in other vertebrate embryos at the same general period of development. (See p. 

 520.) Superficially, therefore, the general structure of the primitive embryonic 

 body of one species resembles that of the primitive embryonic bodies of other 

 vertebrate species. Such comparable conditions of primitive, body-form devel- 

 opment are reached in the 10 to 15-mm. embryo of the shark, Squalus 

 acanthias, of the frog embryo at about 5 to 7 mm., the chick at about 55 to 

 96 hrs. of incubation, the pig at 6 to 10 mm., and the human at 6 to 10 mm. 



2) Larval Body Form. Following primitive body form, the embryo grad- 

 ually transforms into a larval form. The larval form is present in the period 

 between primitive body form and definitive body form. The larval period is 

 that period during which the basic conditions of the various organ systems, 

 present in primitive body form, undergo a metamorphosis in assuming the 

 form and structure of the adult or definitive body form. In other words, during 

 the larval period, the basic or generalized conditions of the various organ 

 systems are changed into the adult form of the systems, and the larval period 

 thus represents a period of transition. Embryos which develop in the water 

 (most fishes, amphibia) tend to accentuate the larval condition, whereas 

 those which develop within the body of the mother (viviparous teleosts, 



