400 GASTRULATION 



process as a whole and bring about the organization of the entire dorsal axial 

 system of notochord, neural tube, somites, etc. In this series of activities, neural 

 plate induction and neural tube formation merely are secondary events of a 

 general organization process. 



A clear-cut distinction should be drawn, therefore, between the action of 

 the dorsal-lip organizer, in its normal position and capacity, and that of an 

 ordinary inductor which induces secondary neural tube development. The 

 characteristics of the primary organizer or organization center of the early 

 gastrula are: 



(a) its ability for autonomous or self-differentiation (that is, it possesses the 

 ability to give origin to a considerable portion of the notochord, pre- 

 chordal plate material, and axial mesoderm of the secondary embryo), 



(b) its capacity for self-organization^ 



(c) its power to induce changes within and to organize surrounding cells, 

 including the induction and early organization of the neural tube. 



As a result of its comprehensive powers, it is well to look upon the organi- 

 zation center (primary organizer) as the area which determines the main fea- 

 tures of axiation and organization of the vertebrate embryo. In other words, 

 // directs the conversion of the late blastula into the axiated gastrular condition 

 — a condition from which the primitive vertebrate body is formed. Induction 

 is a tool-like process, utilized by this center of activity, through which it effects 

 changes in surrounding cells and thus influences organization and differentia- 

 tion. Moreover, these surrounding cells, changed by the process of induction, 

 may in turn act as secondary inductor centers, with abilities to organize specific 

 subareas. 



An example of the ability of a group of cells, changed by inductive influence, 

 to act as an inducing agent to cause further inductive processes is shown by 

 the following experiment performed by O. Mangold ('32). The right, pre- 

 sumptive, half brain of a neurula of Ambystoma mexicanum, the axolotl, was 

 removed and inserted into the blastocoel of a midgastrula of Triton taeniatus. 

 Eight days after the implant was made, a secondary anterior end of an embryo 

 was observed protruding from the anterior, ventral aspect of the host larva. 

 An analysis of this secondarily induced anterior portion of an embryo demon- 

 strated the following: 



( 1 ) The original implant had developed into a half brain with one eye and 

 one olfactory pit. However, 



(2) it also had induced a more or less complete secondary larval head 

 with a complete brain, two eyes, with lenses, two olfactory pits, one 

 ganglion, four auditory vesicles, and one balancer. One of the eyes had 

 become intimately associated with the eye of the implant, both having 

 the same lens. 



