392 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



small evaginations for the formation of the muscles of mouth parts and 

 legs (Fig. 348, mus). 



About the time that the flexure of the embryo occurs the yolk sac 

 separates from the body wall at each lateroventral angle (Fig. 343, sin) 

 forming, a lateral blood sinus, which represents the first anlage of the 

 definitive body cavity or schizocoele. The blood cells in the cavity come 

 from mesoderm cells which some time before were observed on the median 

 line between the coelomic sacs. At this time the coelomic sacs are 

 located in a restricted space in and at the base of the appendages (Figs. 

 343, 346). Later the blood sinus becomes narrower but extends much 

 further toward the dorsal side (Fig. 348). With the extension of the 

 blood sinus the coelomic sac pushes dorsally and ventrally {coel 1, coel 2). 

 At the same time the widely separated nerve ganglia move toward 

 the ventral median line. The middle section of the coelomic sac no 

 longer exists as such, having been wholly converted into leg muscles, 

 dorsoventral muscle strands (Fig. 348, mus), fat (/), and mid-gut peri- 

 toneum. The dorsal and ventral stretching of the sac results in the 

 lumen's becoming reduced and in the walls' becoming thin, the cells 

 assuming a spindle-like character. At the dorsal junction of the visceral 

 and somatic layers of the dorsolateral limb of the sac are the cardioblasts 

 (chl) which are destined to form the heart muscles. The dorsal limbs of 

 the coelomic sacs near the cardioblasts, including their metameric arrange- 

 ment and the septa between the coelomic cavities, are well preserved, 

 although the ventral limbs -of the sacs are in varying stages of modifica- 

 tion. This dorsal portion may be designated as the genital region, for in 

 it the germ cells will later be harbored. The splanchnic mesoderm has 

 grown around the yolk (Fig. 348, splm) as a thin-walled sac, whose growth 

 is independent of and more rapid than the dorsal and ventral growth of 

 the coelomic sacs (Fig. 348, coel). 



The cardioblasts (chl), which are readily distinguishable from adjacent 

 mesoderm cells by their larger size and their oval nuclei, form a longi- 

 tudinal strand of cells extending from the eighth to the twenty-eighth 

 metamere. In the anterior part from the antennal to the maxilliped 

 segments, smaller, less sharply differentiated cells, which Heymons calls 

 "vasoblasts," will form the aorta. The dorsad growth of the dorsal limb 

 of the coelomic sac (coel 1) carries with it the crescent-shaped cardioblasts, 

 which, meeting those from the opposite side, unite to form the heart, 

 the space left between the horns of the crescents forming the lumen. The 

 lumen of the heart is therefore not a part of the coelomic cavity. The 

 heart itself is composed of two layers : the inner layer of transverse muscle 

 fibers derived from the cardioblasts, the outer layer of longitudinal fibers 

 derived from the smaller adjacent mesoderm cells. The heart valves are 

 also derived from the cardioblasts and are approximately segmental in 



