THE ORGANIZATION CENTER 401 



The series of inductive processes presumably occurred as follows: The im- 

 planted half brain induced from the epidermis of the host a secondary anterior 

 end of a neural plate; the latter developed into a brain which induced the 

 lenses, auditory vesicles, etc. from the host epidermis. Thus, the original 

 implant, through its ability to induce anterior neural plate formation from 

 the overlying epidermis, acted as a "head organizer." 



The transformation of the late blastula into the organized condition of the 

 late gastrula thus appears to be dependent upon a number of separate induc- 

 tions, all integrated into one coordinated whole by the "formative stimulus" 

 of the primary organizer located in the pre-chordal plate area of entodermal- 

 mesodermal cells and adjacent chordamesodermal material of the early gastrula. 



2. Divisions of the Primary Organizer 

 The primary organizer is divisible into two general inductor areas as follows: 



(a) the pre-chordal plate of entomesodermal material, and 



(b) the chordamesodermal cells which come to lie posterior to the pre- 

 chordal plate area of the late gastrula. 



The pre-chordal plate is a complex of entodermal and mesodermal cells 

 associated at the anterior end of the notochordal cells in the late gastrula. In 

 the beginning gastrula, however, it lies between the notochordal material and 

 the dorsal-lip inpushing of the entoderm in amphibia, and just caudal to the 

 notochordal area in teleosts, elasmobranch fishes, reptiles, and birds (figs. 

 169; 173 A; 179B; 180B). The chordamesodermal portion of the primary 

 organizer is composed of presumptive notochordal cells and that part of the 

 presumptive mesoderm destined to form the somites. The pre-chordal plate is 

 known as the head organizer, because of its ability to induce brain structures 

 and other activities in the head region. (The use of the phrase head organizer 

 as a synonymous term for pre-chordal plate is correct in part only, for a 

 portion of the anterior notochord and adjacent mesoderm normally is con- 

 cerned also with the organization of the head.) On the other hand, the pre- 

 sumptive notochord with the adjacent somitic (somite) material is described 

 as the trunk or tail organizer (fig. 191G) because of its more limited inductive 

 power. For example, Spemann ('31) demonstrated that the head organizer 

 transplanted to another host embryo of the same age produced a secondary 

 head with eye and ear vesicles when placed at the normal head level of the 

 host. Also when placed at trunk level, it induced a complete secondary embryo 

 including the head structures. However, the trunk organizer is able to induce 

 head and trunk structures at the head level of the host; but in the trunk region 

 it induces only trunk and tail tissues. (See Holtfreter, '48, pp. 18-19; Needham, 

 '42, pp. 271-272; Spemann, '31, '38. The student is referred also to Huxley 

 and De Beer, '34, Chaps. 6 and 7; and Lewis, '07.) 



