240 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



formed, the first pair arising after the formation of the provisional dorsal 

 closure but before blastokinesis. The separation of the outer dorso- 

 lateral end of the somatic mesoderm from the provisional dorsal closure 

 gives rise to the two more or less spindle-shaped lateral sinuses. Later 

 the first pair of lateral sinuses is lost by the breaking away of the pro- 

 visional dorsal closure from the edges of the germ band. Later still, the 

 two lateral blood sinuses unite into a common dorsal blood sinus (Fig. 



Fig. 163. — Locusta. Longitudinal section of aorta (ao) one day before hatching, (ant. 

 CO) Inner wall of antenna! coelomic sac. {hi) Blood cell. (/) Fat. {g. ph) Pharyngeal 

 ganglion, {mus) Muscle, (n) Nerve from brain, {tr) Trachea, (x) Lateral wall of 

 antennal coelome forming fatty tissue (/) . 



159), and finally the true heart cavity arises by the fusion of the 

 cardioblasts from either side. 



About one day after blastokinesis the provisional mesodermal mid- 

 gut layer is invested externally by a thin mesentery (Fig. 164). The 

 latter lies close to the former, nevertheless is separate from it except 

 in the middorsal region. The narrow space between is the circum- 

 intestinal blood sinus. The outer mesentery which arises from the ven- 

 tral portion of the splanchnic mesoderm soon afterward disappears. 



The pericardial cells first make their appearance about a day after 

 blastokinesis, forming an irregular string of large round cells on each 



