HEART. 339 



the germ-band, this rudiment shifts more and more towards the 

 dorsal side. It is directly connected with the wall of the primitive 

 segments (Figs. 170 and 171), and indicates the junction of the 

 dorsal with the lateral Avail of the coelomic sac. According to 

 Korotneff (Xo. -17), whom we have to thank for the first detailed 

 account of the development of the heart in the Insecta, the cardio- 

 blasts are derived from the wall of the primitive segments. 



In Qryllotalpa, the form described by Korotneff, the condition 

 is in many respects peculiar. The formation of the dorsal organ is 

 here introduced, in the way described above (p. 301, etc.), by the 

 rupture of the embryonic envelopes. The serosa contracts to form a 

 thickened plate (Fig. 167 A, rp), of which the very degenerate 

 amniotic folds appear as a lateral appendage (am), the whole being 

 far removed dorsally from the edges of the germ-band (*x-y*) 

 (cf. Fig. 150 C, p. 306). The interval between the rudiment of the 

 amniotic fold and the lateral edge of the germ-band (*x-y*) is 

 occupied by an epithelial lamella (I) in which we recognise the 

 former amnion. This lamella is not closely applied to the yolk, 

 but is separated from it by a spacious blood-lacuna (bs), in which 

 •can be seen numerous blood-corpuscles that have immigrated from 

 the mesoderm. The cardioblasts which are derived from the wall of 

 the primitive segment (us) have become arranged to form a channel 

 (gr) on each side, and thus surround the lower part of the blood- 

 sinus. 



As the circumcrescence of the food-yolk by the germ-band pro- 

 gresses, after the invagination and degeneration of the dorsal plate 

 has taken place, the two blood-lacunae fuse together dorsally to form 

 one lacuna (Fig. 167 B, bs). This now represents the rudiment of 

 the lumen of the heart. The two vascular channels, moving towards 

 «ach other until they come into contact, form, by fusing together, the 

 wall of the heart (Fig. 167 C, r, and Fig. 172, Ji, p. 346). The 

 venous ostia arise, according to Butschli (Xo. 11), as paired invagina- 

 tions of the lateral walls, in the base of each of which a slit develops. 



The rudiment of the heart, as we have seen, is intimately connected 

 with the primitive segments. The lateral wall of the primitive 

 segments, after giving off the elements of the somatic mesoderm, 

 gives rise to an epithelial plate which represents the first rudiment 

 of the pericardial septum or dorsal diaphragm (Fig. 167 A-C, dd : 

 Fig. 170, p. 341; Fig. 171, p. 345, and Fig. 172, ps, p. 346). As 

 soon as the two halves of the rudiment of the heart have united in 

 the middle line, the two halves of the pericardial septum also become 



