The Development of Theridiiim. 343 



iiing of the pericardial cavity. With the formation of the heart most 

 of the large blood colls become placed within its cavity. The heart 

 is formed very rapidly as a continnons dorsal tube extending from. 

 the brain region (cephalic artery) caudad to the caudal lobe; the 

 dissepiments (transverse spaces between segments) constitute its 

 earliest vessels or ostia, and there are at reversion seven or eight 

 pairs of these. 



Claparede (180::^) and Salonsky (1871) found the heart to be 

 solid at first, and its core later changing to blood corpuscles. Balfour 

 (1880) reached the same conclusion, and stated the heart to be meso- 

 blastic "formed before the dorsal mesoblast has become differentiated 

 into two layers." The cells which I have described as blood cells he 

 considered derivatives of yolk cells. Locy (1886) was "convinced 

 that it is not, as Balfour states, developed from a solid cord of cells, 

 but from the dorsal limb of the upgrowing mesoderm, and that its 

 dorsal wall is closed first." He maintained that the aorta developed 

 later from the mesenteron, and that the blood cells are derivatives of 

 yolk cells. Morin (1887) found large extraembryonic cells ("links 

 und rechts vom Keimstreifen") between ectoblast and yolk that be- 

 come blood cells, and he supposed they arise from mesoblast. With 

 the dorsal growth of the mesoblastic somites at re\'ersion these large 

 cells come to lie dorso-median, and the heart wall is formed by the 

 adjacent mesoblast, the heart cavity being archicoelic. Schimke- 

 witsch (1887) held the blood cells are derivatives of "secondary 

 entoblast," which forms from yolk cells that become round, reach the 

 surface of the yolk and get into coelomic cavities ; ultimately these 

 become placed within the heart, the development of which is as 

 Morin described. Kishinouye (1890) corroborated Morin's descrip- 

 tion of the heart formation, and found (1894) that "the number of 

 the slits in the adult heart shows approximately the number of the 

 segments which took part in the formation of the heart" ; namely, 

 the second to the fifth abdominal segments inclusive. 



The heart cavity is archicoelic, its wall mesoblastic, and the peri- 

 cardial cavity coelomic, as Morin, Schimkewitsch and Kishinouye 

 have insisted. But all the writers have failed to observe the very 

 characteristic formation of blood cells from the extraembryonic ecto- 

 blast. 



