PROBLEMSOFORGANIZATION 1 99 



problem. Holtfreter ('43b, '44) has recently studied the problem in 

 the amphibian and has shown that many of the movements which we 

 associate with fertilization can occur in the unfertilized ^gg. The 

 surface movements of pigment and the initial cortical relationships of 

 the plasmogel simulate many of the relations of the fertilized egg 

 reactions. While some of these reactions were observed previously, the 

 various components have not been so completely traced and their sig- 

 nificance correlated as in the study referred to. In a second study he has 

 shown the contours of isolated endodermal cells taken from the am- 

 phibian gastrula. The blastoporal cells can be identified and spread in 

 the direction of their former proximal-distal axis. They exhibit a 

 definite precocious polarization whereas the non-polarized cells spread 

 in a constant radiating reaction. It must be remembered that these cells 

 are taken from the early gastrula, just as the endoderm is being invagi- 

 nated. There are two types of cells in the early gastrula, those which are 

 polarized and those which are nonpolarized. Nieuwkoop (see above) 

 assigns to the endoderm the role of generally organizing the mesoderm 

 which later is going to be found in the dorsal mesenteries and in a 

 primordial germ ridge with the general sensitivity for germ cell forma- 

 tion, acting therefore as a primary organizer of this tendency on the 

 part of certain of the mesoderm cells. If this be so, the polarization 

 which Holtfreter has observed has an added meaning, for it is the 

 polarized cells which exert the inductive influence. 



In the study of the mechanics of gastrulation Holtfreter ('43a, '44b) 

 has demonstrated the influence of the surface coat which is found upon 

 the cells. Uncoated endoderm is incorporated into an endodermal sub- 

 strate, whereas a substrate of gill ectoderm will spread over a graft of 

 blastoporal endoderm. Endoderm will pentrate through a hole in the 

 epidermis and pass into an endodermal substrate. This and other factors 

 which are consistent with the formation of endoderm are treated by 

 Holtfreter and material which possesses a surface coat has a definite 

 affinity to invaginate into the substrate. However, when mesoderm is 

 placed upon a piece of glass and two pieces of ectoderm are placed in 

 contact with it, the ectoderm moves up along the outside of the meso- 

 derm and encloses it. In this case the mesoderm is uncoated, while the 

 ectoderm is coated. A reverse effect occurs when a coated piece of the 

 marginal zone is placed in contact with host ectoderm and invaginates 

 under the host ectoderm, which again is a covered type of tissue. In this 

 case we have two covered pieces moving together with the marginal 

 zone to form a type of archenteron. There seems to be no consistency 



