i'EOM EGG TO INSECT SNODGRASS 



41] 



the heart suck the blood into it through its lateral openings, or ostia 

 (Ost), drive it forward, and then expel it into the head. From here 

 it percolates backward between the organs of the body cavity, goes 

 into the dorsal sinus, and finally reenters the heart. The number of 

 chambers in the heart varies in different groups of insects. Usually 

 there is a pair of ostia to each chamber 

 and one pair of " wing muscles," but ^ 



these features, likewise, are not constant 

 in different insects. 



THE REPRODUCTIVE ORGANS 



The internal parts of the reproductive 

 organs, in which the germ cells are lodged 

 and in which these cells undergo their 

 development into the spermatozoa or 

 eggs, according to sex, appear first as 

 thickenings of the upper parts of the 

 inner mesoderm la3'^ers in the abdominal 

 region of the body. A strand of cells 

 proceeds rearward from each, which be- 

 comes the duct, opening usually into a 

 single, median outlet tube formed as an 

 ingrowth from the ectoderm near the 

 posterior end of the body. The germ 

 cells (fig. 10 B, C, GCls), during the 

 growth of the embryo, wander about for 

 some time in the other tissues, but at last 

 they come to their abiding places in the 

 reproductive organs (figs. 21, 27, Rep.). 



THE YOUNG INSECT 



Ao 



Mel 



Fig. 26. — Diagram of the typ- 

 ical structure of the heart 

 and diaphragm of an insect, 

 and the cour.se of the circu- 

 lating blood (indicated by 

 arrows) 



Ao, aorta, or anterior tubu- 

 lar part of heart without lat- 

 eral openings ; Dph, membran- 

 ous diaphragm ; Tit, anterior 

 three chambers of heart ; McJ, 

 muscles of diaphragm, attached 

 laterally to body wall ; Ost, 

 ostium, or lateral opening into 

 heart chamber. 



The story of insect embryology, as 

 briefly outlined in the preceding de- 

 scriptions, shows how a complex animal 

 comes into existence from the simplest 

 possible beginning. First, a group of 

 primitive body cells is formed by the 

 repeated division of a fertilized Q,gg, 

 nucleus. These cells continue to multiply and their increasing 

 descendants segregate themselves into other groups, which subdivide 

 again into minor groups, until there is a group of cells representing 

 each tissue of the adult animal. Compound organs are formed 

 either from specialized cell groups all of the same tissue, or by the 

 union of parts derived from cell groups of independent origin. 



