PROBLEMS OF ORGANIZATION 195 



of the same magnitude, with the possible exception of the heavy aggre- 

 gates which are found in the lining of the alimentary tract. This is the 

 last material which is effective in differentiation and is really the begin- 

 ning of the circulating nutritive factor. 



Further study of these stages with the application of the recently 

 developed histochemical tests is necessary for the interpretation of those 

 important agents that are deposited in the various substrates which are 

 present in yolk. Since this is the material which by chemical transference 

 becomes the tissue of the embryo, it is vital that the changes in it be 

 clearly analyzed in order that the relation between chemical changes and 

 the mechanics of development be adequately understood. 



Organizational Dependencies 



When we come to the study of relationships of organization in sepa- 

 rate systems, we begin to perceive how far organization enters into the 

 process and how much is intrinsic to cellular self-determination. Holt- 

 freter's ('39) early studies showed that tissues, when in combination, 

 produce far more than the individual elements by themselves even when 

 acting under an organization complex. When connective tissue is pres- 

 ent the cells tend to form more than when endoderm or ectoderm is 

 present by itself. Stableford ('39) found that it was difficult to isolate 

 the internal portion of the system without the necessary adjunct of 

 some of the mesoderm which is going to form connective tissues or 

 muscle layers. He indicates that the endoderm also is a source of meso- 

 derm formation and that isolated endoderm developing in Holtfreter's 

 standard solution can be shown to form mesoderm.* 



Nieuwkoop ('46), however, has performed this experiment in re- 

 verse and has isolated the ectoderm-mesoderm complex from the endo- 

 derm. While there are definite deficiencies in the mesodermal structures, 

 there is an attempt by the organism to produce a complete somatopleure- 

 splanchnopleure combination, but the mesodermal structures are not 

 acted upon by the endoderm which has suffered early removal in the 

 early neurular stage. In these experiments Nieuwkoop has found that 

 two complete sets of limbs will evolve, one of them in the normal loca- 

 tion and from the normal structure of a somatopleure, the other formed 

 from the splanchnopleuric portion of the mesoderm at the same level 

 as the somatopleuric limbs. This is the strongest index that we have had 



* Stableford in a subsequent study, as yet unpublished, concludes that marginal zone 

 is the sole source of mesoderm and therefore the endodermal capacity for mesodermal 

 origin must be subjected to further test. 



