MESODERMAL DERIVATIVES 119 



cells that are set free Toyama (1902) interprets as blood cells. Will 

 (1888), Schwangart (1904), and Nusbaum and Fulinski (1906) as well as 

 Hirschler regard the blood cells as entodermal. Recent workers, though, 

 are generally agreed that the blood cells arc mesodermal and for the most 

 part arise from the midventral strand. 



PERICARDIAL CELLS 

 The pericardial cells were first described by Wielowiejski (1883). 

 They are associated with the fat body and, as the name indicates, found 

 near the heart. They are readily distinguishable from the fat cells in 

 the pericardial chamber. Heymons (1895a) found that in the Blattidae 

 and Gryllidae they originate from the so'matic layer of mesoderm. In 

 Forficula they do not attain their characteristic appearance until post- 

 embryonic life. In Locusta, according to Roonwal (1937), they first 

 make their appearance in the embryo about a day after blastokinesis, 

 arising from the somatic mesoderm abutting on the ventrolateral aspect 

 of the heart. Wiesmann (1926) states that in the walking stick Carausius 

 these cells migrate out as free cells from the pericardial septum, or 

 diaphragm. 



THE PARACARDIAL CELL STRAND 

 The term "paracardial cellular cord," or "cell strand," was proposed 

 by Heymons (1895a) for a paired structure that he found in Forficula 

 at the sides of the dorsal blood vessel in metameric arrangement between 

 the wing muscles of the heart in the pericardial septum. The cells of this 

 strand are conspicuous by reason of their size and pale color even before 

 revolution. They arise in the center of the individual body segments 

 from the dorsal parts of the coelomic sacs and retain their peculiar char- 

 acter throughout their entire embryonic development. Heymons points 

 out the resemblance of this cell strand to the "guirland cell strand" 

 described by Weismann (1864) which he discovered in the larvae of 

 muscoidean flies. Kowalewsky (1886) called attention to the physiologi- 

 cal equivalence between the guirland cell strand and the pericardial 

 cells. Just as with the guirland strand so is the paracardial strand 

 especially well developed in embryonic and larval life of Forficula; in 

 adult life it has become less conspicuous. Histologically the cells of the 

 paracardial strand and the pericardial cells in Forficula appear to be 

 identical, and for this reason Heymons feels justified in considering the 

 paracardial and the guirland cell strands as similar. 



The paracardial cell strand is lacking in both the Blattidae and 

 Gryllidae but present in Chalicodoma (Carriere and Burger) and in the 

 ants (Strindberg, 19136). In the dorsal diaphragm (pericardial septum) 

 of Apis, Nelson (1915) found pale lenticular cells whose size approximates 



