PROBLEMS OF ORGANIZATION 205 



method is just as efficient in its complex potentiality as is that of the 

 amphibian. 



There are four types of movement that one may designate as occur- 

 ring in the Drosophila egg. The first are general movements involving 

 nuclei and their peripheral distribution; the second, specific movements 

 of cells and of layers; the third, specific movements of cell aggregates 

 which later will form organs within the organism ; and finally the gen- 

 eral movements involved in the shortening of the embryo, which con- 

 tinue within the nervous system and result in the high degree of con- 

 densation characteristic of that system in the higher Diptera. The 

 condensation of the nervous system continues after the germinal band 

 has begun to proliferate upward along the sides to surround the intes- 

 tinal canal, and nerve fibers accompany the migrations of the mesoderm. 

 In this it is not unlike the vertebrate system. It differs from it, however, 

 in that (during the process) it sends proportionately very much longer 

 nerve fibers in connection with the contraction of the central system it- 

 self. The localization of the various parts and their relationships to 

 future organization are clearly shown in Poulson's ('45) rather re- 

 markable investigation of this form. 



The movements described above may be reduced to a rather simplified 

 form in the following classification. 



Classification of Movements in Drosophila 



A. General movements (early egg) 



1. Nuclear 



a. early distribution 



b. peripheral migration 



2. Cytoplasmic 



a. early (with nuclei) 



b. disengagement from yolk 



c. furrowing of peripheral cytoplasm 



d. sync>1:ial layer to cellular layer 



B. Special movements 



1. Cellular migrations 



a. endoderm in gastrulation 



b. pole cells 



c. neuroblasts 



2. Aggregate movements 



a. mesoderm in gastrulation 



b. proctodaeum 



