206 J. S. NICHOLAS 



c. stomodaeum 



d. mid-gut 



e. nervous system 



C. General embryonic (germ band and later) 



1. Surface movements 



a. germ band extension 



b. germ band shortening 



c. dorsal closure 



2. Invagination 



a. involution of head 



The story of the movements of parts during organization is not nearly 

 so complete in the amphibian (Gillette ('44)), Here a similar classifi- 

 cation scheme might be evolved, but there are tremendous gaps in the 

 analysis of the various types of movement which occur. There are many 

 observations which indicate that movements occur, but whether these 

 are molecular, particulate, cellular, or aggregate is in general not noted, 

 and accurate measurements of the amount of movement and the forces 

 necessary for the final position are not yet available. Gillette ('44) has 

 investigated the neurula and has given the most concrete example of 

 mass movements of cell aggregates in the formation of the neural plate. 

 There is a dorsal concentration of cells which formerly occupied a lateral 

 position. At the same time there is a smaller but definite relocation of 

 some of the ventral cells which move into the lateral position. There is 

 a loss of 1 1 % of the total ectoderm from the lateral lunes and 7% from 

 the ventral lunes in providing the ectodermal material which is taken 

 over in the dorsal region. 



Chemical Organization 



The work dealing with the chemistry of the neural organizing sub- 

 stance, or substances, beginning with Spemann, Fischer, and Wehmeier 

 ('33) and continuing to the present (for reviews see Needham ('42) 

 and Boell ('42)) has shown how many different substances may cause 

 this effect. No particular one, however, can be said to be dominant in 

 the reaction. The analyses for glycogen in different regions by Woerde- 

 man ('33), Heatley ('35) and Heatley and Lindahl ('37) showed that 

 glycogen disappears in the dorsal lip during gastrulation. This fact has 

 been employed by Boell ('42) as showing the significance of the reac- 

 tions in association with the dorsal lip and carbohydrate metabolism. 

 Boell (loccit.) implies that the quantitative differences in metabolism 



