484 PATTERNS AND PROBLEMS OF DEVELOPMENT 



Pieces of extraembryonic ectoderm, presumptive epidermis, and pre- 

 sumptive axial or lateral mesoderm of primitive-streak stage implanted 

 under the primitive streak of the same stage are all capable of developing 

 into neural tissue. Their anteroposterior axes may be altered or reversed 

 by the host, but dorsiventrality persists (Abercrombie, 1937). The neural 

 development of these implants is considered to be due to induction by the 

 host streak. The polarities of pieces of primitive-streak pieces implanted 

 beneath primitive streak may also be reversed by the host and their re- 

 gional character altered to conform to host-level. In some cases the host 

 axis shifts to one side of the implant, and the latter induces either extra 

 neural tissue or a new axis, or the implant may be incorporated in the host 

 body (Abercrombie and Waddington, 1937). By implantation into the 

 primitive streak presumptive ectoderm may be converted into mesoderm 

 (Waddington and Taylor, 1937). In these cases the effect of the host on 

 the implant is evident in varying degree. The implant may retain its in- 

 dividuality and induce, or be altered as to polarity and body-level, or be- 

 come a part of the host body. 



An interesting apparent induction by the entodermal axis has been 

 obtained by separating epiblast and entoderm in early primitive-streak 

 stages of duck and chick embryos and replacing them with longitudinal 

 axes opposed (Waddington, 1933a). Effects on development of this pro- 

 cedure vary. In cases of axial induction two embryos may develop with 

 heads together and longitudinal axes opposed, one representing the epi- 

 blast axis, the other apparently induced by the entodermal axis; or the 

 epiblast axis determines an embryo, the entodermal axis a transitory prim- 

 itive streak. In still other cases the original primitive streak disappears, 

 and the embryo determined by the entodermal axis persists. Even when 

 a new embryonic axis is not induced, the original axis may be inclined to- 

 ward the direction of the entodermal axis or be semicircular, or the em- 

 bryo may be very short. Waddington holds that the entoderm merely in- 

 duces the cell migrations that result in development of the primitive 

 streak but admits that a gradient system or other axiate pattern is neces- 

 sary to account for the orderly character and definite directions of the mi- 

 grations. However, it does not appear from the data that the entodermal 

 axis differs essentially in its inducing action from implanted parts of the 

 primitive streak. It is apparently a rather effective inductor, for it is able 

 to determine a new polarity opposed to the original. That this polarity 

 results from the imposition on the epiblast of a new gradient pattern cor- 

 responding to the entodermal pattern seems probable. The persistence of 



