118 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



finally from the latter the definitive heart cavit}^ arises by the fusion of 

 the cardioblasts of either side. 



The dorsolateral wall of the somatic mesoderm, reduced to a thin 

 sheet as it grows dorsad, forms the dorsal diaphragm (pericardial septum). 

 Shortly before emergence, fan-shaped bundles of muscle fibers appear 

 in this septum attached at one end to the body wall and at the other 

 to the ventral wall of the heart. In Carausius the diaphragm is originally 

 laid down as a two-layered sheet. 



The dorsal aorta is formed from the median walls of the antennal 

 coelomic sacs and not from cardioblasts. Hirschler contends that in 

 Donacia the aorta develops from the intercalary coelomic sacs. Paterson 

 (1932) reports a similar condition in the chrj^somelid Euryope. In 

 Silpha, one of the carrion beetles, however, Smreczynski states that the 

 aorta arises from the median walls of the antennal sacs, the suspensory 

 ligaments being formed from the remaining parts of the sacs. Likewise 

 in Apis, Pieris, Forficula, Eutermes, Carausius, and other Orthoptera 

 the aorta originates from the antennal sacs, this appearing to be the 

 normal method of formation. For an account of the development of the 

 heart of the locust the reader is referred to Chap. XV. 



BLOOD CELLS 



Earlier writers have not been in agreement as to the mode of origin 

 of the blood cells which have been variously described as arising from 

 the yolk cells, from the cells of the serosa, from the walls of the heart, 

 from undifferentiated mesoderm cells, from the subesophageal body, 

 from the entoderm, and even from the ectoderm. 



Hirschler (1909) states that the blood cells arise from the middle 

 strand that lies between the two rows of coelomic sacs. In his opinion, 

 however, the middle strand, whose cells in many insects aid in the forma- 

 tion of the mid-gut epithelium, is entodermic, and therefore so also are 

 the blood cells. In Isotoma Philiptschenko (1912) likewise derives both 

 blood cells and mid-gut epithelium from the middle strand. Leuzinger 

 and Wiesmann (1926) state that the blood cells of Carausius come from 

 a secondarily reconstructed middle strand which like the first middle 

 strand also gives rise to mid-gut epithelial cells. They, however, regard 

 the blood cells (haemocytes) as mesodermal or possibly intermediate 

 between mesoderm and entoderm. In Apis the blood corpuscles are 

 described by Nelson (1915) as large round cells approaching in size the 

 oenocytes. Both nucleus and cytoplasm are pale, the latter being 

 much vacuolated and frequently enclosing deeply stained granules. 

 These cells, regarded as mesodermal, arise in the immediate vicinity of 

 the mid ventral line and are to be found in the epineural sinus. With the 

 disintegration of the subesophageal body in the silkworm embryo the 



