204 J. S. NICHOLAS 



against the mesoderm. The neuroblasts contain a large amount of cyto- 

 plasm and can be recognized as constituents which are distributed along 

 the entire length of the germ band. The neuroblasts immediately un- 

 dergo the characteristic unequal divisions which produce the ganglion 

 cells. The entire nervous system is originally bipartite, a condition re- 

 flected in its later bilateral structure. The differentiating ganglion cells 

 send out processes from the nerve cord into the mesoderm and these ex- 

 tend along with the mesoderm into the lateral areas, becoming the seg- 

 mental nerves. 



Lateral ventral cells which remain at the surface after the neuroblasts 

 have moved to the interior become hypoderm cells and later produce the 

 larval integument, except in certain localized regions where they give 

 rise to salivary plates (later invaginated and pulled into the interior 

 through the stomodaeum) and the eleven pairs of tracheal pits which 

 give rise to the major part of the tracheal system. 



By the middle period of development ( 10-12 hrs.) the principal larval 

 structures, or their rudiments, are all laid down. The shortening of the 

 germ band brings many cells back to somewhere near their original po- 

 sitions, but there are new shifts as the dorsal closure of the embryo is 

 completed and segmentation proceeds. Special movements are concerned 

 in the involution of the head region, which is characteristic of the larvae 

 of higher Diptera. The imaginal discs begin to make their appearance 

 only a short time before the hatching of the larva from the egg (at 20- 

 24 hrs. after fertilization, depending on the temperature). 



The movements within the embryo of Drosophila are more complex 

 than in the amphibia where they are limited to two general types of 

 movement. The first of these is found in the egg just after fertilization, 

 continuing through the segmentation stages and blastulation, the second 

 comprises the later general movements, resulting in the formation and 

 distribution of the layers and the formation of the nervous system. Spe- 

 cific movements of the type performed in Drosophila seldom occur. In 

 Drosophila the specific movements apparently are those of aggregates 

 of cells which tend to adhere to each other and form functional masses 

 from which are derived the specialized tissues and organs of the embryo. 

 An amphibian would produce its nervous system if it were similar to a 

 Drosophila by the creation of more parts from which certain areas 

 would be deleted. Drosophila, however, has its parts fully formed, its 

 ganglion cells sending their processes out to the periphery, and then a 

 contraction or aggregation of the different parts of the system occurs, 

 with a degeneration of some of its parts to form a smaller unit. This 



