198 J. S. NICHOLAS 



deletion and transplantation methods to strengthen the validity of his 

 interpretation. When a sector of more than 90 degrees was removed 

 from the embryonic area, no embryo formed. In the absence of embryo 

 formation, however, the blastoderm gradually expanded to cover the 

 yolk. When a 70-degree sector was removed from the embryonic area, 

 regulation occurred and embryo formation could still be accomplished by 

 what remained of the blastoderm. When an embryonic sector of 45 

 degrees was removed from the gastrula stage, the chorda and anterior 

 somites were lacking in some of the embryos which developed. No such 

 gradient has been observed in the Ggg of Fundulus (cf. Oppenheimer, 

 supra). However, the experiments are not exact counterparts of those 

 on Salmo. Small portions only of the germ ring were removed from 

 regions 90 degrees or 180 degrees away from the embryonic axis and 

 these were grafted into embryonic shields, or into the extraembryonic 

 membranes of an embryo of the same age as the donor. Such grafts 

 implanted on the extraembryonic membrane will differentiate any struc- 

 tures except epidermis, blood cells, and chromatophores. In contrast, 

 grafts from the 90-degree and the 180-degree germ ring of gastrulae 

 of various stages implanted into the shield differentiated without excep- 

 tion provided their cells were incorporated below the epidermis. The 

 nature of the structures differentiated bears no relationship to the sorts 

 of the graft ; head, trunk and tail structures were formed in graft from 

 both 90 degree and 180 degree regions of the germ ring. In some cases 

 grafts formed structures characteristic neither of the region of the host 

 to which they were transplanted nor of the regions of the embryo for 

 which the graft itself was originally destined. 



Independent Movements 



Movements of parts within the egg at early stages are largely deter- 

 mined by the physical reactions of the constituents. During later stages 

 the simpler physical determinants are obscured by specific and general- 

 ized processes of growth and differentiation. These processes are chem- 

 ical and distributional in nature. It has fallen to the descriptive embry- 

 ologist to relate the surface reactions which are immediately visible, 

 the analysis of the underlying orientations and their role in organization 

 as well as their later definitive action in aligning spatially localized parts 

 or organizers and relating these two components is a fundamental 

 problem of embryology which has so far gone unsolved. 



There are many observations and partial attacks upon phases of this 



