PROBLEMS OF ORGANIZATION 20I 



portion of the gastrocoelic floor by an inversion of the blastocoelic floor 

 in the region in which the heart later forms. The gastrocoeHc invagina- 

 tion extends forward to the region of later liver formation. These 

 observations indicate mass movements of material which are responsible 

 for regional localization of determination reactions. This is corrobo- 

 rated by Nieuwkoop's results with a slightly different staining method 

 applied to different stages. 



Mass Movements in Organization 



Two examples of how mass movements are effective in organizational 

 phenomena will suffice. The first is the case of localization in the chick, 

 in which the differences between the older concept of the regional 

 differentiation of the parts of the blastoderm can be contrasted with the 

 results as secured from experimental data." 



The His concept of the chick blastoderm was that of a central axial 

 localization in which the regional localizations are arranged in linear 

 order. The Rudnick diagram (even with the necessary modifications 

 imposed by Spratt's work) deserts the axial type of linear arrangement 

 in favor of a distribution of potencies for the formation of the parts 

 of the organism. Thus one finds heart, thyroid, liver, and chorda not 

 spatially localized in linear order but originally as potency areas which 

 later join to express the final types of specific organization. 



The second example will be presented in a more complete form, for it 

 is the rather complete story of the localizations anticipatory to organiza- 

 tion in Drosophila.^ 



The egg nucleus lies close to the dorsal side of the egg; meiosis leads 

 to formation of three polar nuclei which fuse to form a residual body 

 under the dorsal surface near the female pronucleus. Following fertiliza- 

 tion the zygote nucleus divides a number of times and the nuclei are dis- 

 persed throughout the deutoplasm of the egg mass. The first migrations 

 or movements within the Drosophila egg are those towards the posterior 

 region of the egg where the pole cells later form. Two to five pole cells 

 are formed, each one of them a direct descendant of one of the nuclei 

 which lay originally within the deutoplasmic mass. These subsequently 

 divide to form a total of about thirty pole cells. After a latent period 



2 Since the above comparison was made, Spratt ('46) has published his results with 

 carbon particles employed to study the movements of parts of the chick blastula. The 

 Rudnick diagram is therefore now subject to modification, but in the main shows the 

 cardinal localization of parts in the chick. 



3 For this information I am deeply indebted to my colleague, Dr. Poulson. 



