PHYLUM ARTHROPODA 



247 



from this sympathetic system pass chiefly to the dorsal vessel 

 and the tracheal system. 



Embryology. — The fertilized egg nucleus of insects undergoes 

 several divisions and many of the resulting nuclei migrate from 

 the yolk mass to the superficial layer of the cytoplasm. The 

 layer of cytoplasm containing these nuclei undergoes cleavage 

 and there is thus formed a layer of cells surrounging a central 

 yolk mass (Fig. 8). This is essentially a blastula stage in devel- 

 opment, and the cell layer is fre- 

 quently termed the blastoderm. 

 Those nuclei which remain within the 

 yolk become surrounded by cyto- 

 plasm and are designated as the 

 yolk cells. The blastoderm becomes 

 thickened in one region and forms 

 the germ band from which the ven- 

 tral surface of the embryo later 

 develops. The course of this develop- 

 ment follows two different paths in 

 different insect groups. These are 

 known as the overgrown and the 

 invaginated types of development. 



The former of these involves much g'""^''^ ^^p^ of insect develop- 



ment. ,4, amniotic folds start- 



the less complicated narration. The 

 germ band (Fig. 124) sinks below the 



A 



Fig. 124. — Two diagrammatic 

 sagittal sections to show over- 



ing to cover the germ band 

 (cross-lined) ; B, amniotic folds 

 completed. {From Korschelt and 



surface of the surrounding blastoderm Hcider). 

 to form a groove. As this groove 



deepens, the walls of the blastoderm fold up over the germ 

 band. When the folds of the blastoderm meet, they fuse and the 

 double wall thus formed encloses a cavity known as the amniotic 

 cavity, which lies between the germ band and the before-men- 

 tioned double wall. The outer layer of cells, which comprises 

 the outer margin of the double wall, is termed the serosa and is 

 directly continuous with the blastoderm surrounding the yolk. 

 The inner cell layer, which lines the amniotic cavity, is termed 

 the amnion. The cells of the germ band comprise a layer of 

 ectoderm adjacent to the amniotic cavity, and underlying this 

 ectoderm is a mass of cells which represent both the entoderm 

 and the mesoderm of the embryo. The surface of this germ 

 band, which represents the ventral surface of the developing 

 insect, becomes traversed by a series of transverse grooves to 



