ORIENTATION 



The embryonic period of development in all vertebrate species may be 

 resolved into three distinct phases: 



a. An early embryonic period which begins at the time when the egg starts 

 to develop and which reaches its culmination when the embryo has attained 

 the state of primitive, generalized body form (see Chaps. 10 and 11, and 

 fig. 255). 



b. A period of transition then follows during which the structural condi- 

 tions prevalent in primitive body form are transformed into the morphology 

 present in definitive body form. Definitive body form is reached when the 

 embryo assumes a general resemblance to the adult form of the species. The 

 changes described in Chaps. 12-20 are concerned to a considerable extent 

 with this phase of development. 



c. The late embryonic period. This phase of development comprises the 

 changes which the embryo experiences for a time after it has achieved definitive 

 body form. In the human embryo, it includes several months of fetal growth 

 in the uterus, and in the chick it is of about a week's duration continuing 

 from day 14 of incubation to the time of hatching around day 20. In the frog 

 it is a brief period during the close, and possibly shortly after, metamorphosis. 



The period of transition may be regarded as the larval period of develop- 

 ment. If so conceived, two types of larval forms exist, namely ( 1 ) free-living 

 larval forms such as the frog tadpole in which the body structures are adapted 

 to a free-living existence outside of protective embryonic structures, and (2) 

 non-free-living larval forms in which the larval or transitional period is passed 

 within the confines of covering egg membranes or within the protective tissues 

 of the female or male parent. Free-hving larval forms include Amphioxus, 

 most fishes, and amphibia, while some fishes and all reptiles, birds, and mam- 

 mals may be regarded as having a protected larval existence. 



III. Summary of Developmental Phenomena Associated with the Life 

 of an Individual Vertebrate Animal 



A. Period of Preparation 



During this period the parents are prepared for reproduction and the repro- 

 ductive cells or gametes are elaborated. 



B. Embryonic Development 

 1. Early embryonic period 



This period begins with fertilization of the egg and ends with the develop- 

 ment of primitive embryonic body form with its basic conditions of the various 

 systems. The basic or group condition of a particular vertebrate organ-system 

 is that stage of development of the system when it possesses structural features 

 common to all embryos of the vertebrate group. When the common or primi- 



