BASIC HOMOLOGY OF ORGAN SYSTEMS 545 



erism (branchiomerism). However, all these segmental structures serve a 

 definite end. In other areas, head development proceeds in a manner which 

 obscures segmentation, for the probable reason that segmentation does not fit 

 into the developmental pattern which must proceed directly and precociously 

 to gain a specific end dictated by problems peculiar to head development. 



(Note: For a critical analysis of the supposed facts in favor of segmentation, 

 together with a marshaling of evidence against such an interpretation, consult 

 Kingsbury and Adelmann ('24) and for a favorable interpretation of the seg- 

 mental nature of the head region, see Goodrich ('18) and Delsman ('22). 

 Figure 253 is taken from Goodrich ('18), and the various structures which 

 favor a segmental interpretation of the head region are shown.) 



G. Basic Homology of the Vertebrate Organ Systems 



1. Definition 



Homology is the relationship of agreement between the structural parts of 

 one organism and the structural parts of another organism. An agreeable 

 relationship between two structures is established if: 



( 1 ) the two parts occupy the same relative position in the body, 



(2) they arise in the same way embryonically and from the same rudi- 

 ments, and 



(3) they have the same basic potencies. 



By basic potency is meant the potency which governs the initial and funda- 

 mental development of the part; it should not be construed to mean the 

 ability to produce the entire structure. To the basic potency, other less basic 

 potencies and modifying factors may be added to produce the adult form of 

 the structure. 



2. Basic Homology of Vertebrate Blastulae, Gastrulae, and 

 Tubulated Embryos 



In Chapters 6 and 7, the basic conditions of the vertebrate blastula were 

 surveyed, and it was observed that the formative portion of all vertebrate 

 blastulae presents a basic pattern, composed of major presumptive organ- 

 forming areas oriented around the notochordal area and a blastocoelic space. 

 During gastrulation (,Chap. 9), these areas are reoriented to form the basic 

 pattern of the gastrula, and although round and flattened gastrulae exist, these 

 form one, generalized, basic pattern, composed of three germ layers arranged 

 around the central axis or primitive notochordal rod. Similarly, in Chapter 

 10, the major organ-forming areas are tubulated to form an elongated embryo, 

 composed of head, pharyngeal, trunk, and tail regions. As tubulation is ef- 

 fected in much the same manner throughout the vertebrate series and as the 

 pre-chordal plate mesoderm, foregut entoderm, notochord, and somitic meso- 



