88 



ARACHNIBA. 



Fig. 46.— Cross-section through the abdomen of an embryo 

 of Pholcus phalangioides (after Mokin). bg, ventral chain 

 of ganglia; bl, blood-corpuscles; d, yolk; dz, yolk cells; 

 h, heart; so, somatic, sp, splanchnic niesoblast; sp.w, 

 spinning mammillae. 



So long as the origin of the isolated cells lying between the 

 ectoderm and the yolk is not definitely established, we may regard 



them asmesoderm-cells, 

 and Ave are especially 

 inclined to consider 

 them as derivatives of 

 the yolk-cells from a 

 comparison with simi- 

 larly related cells found 

 in the Vertebrata, which 

 are there undoubtedly 

 derived from the yolk. 

 These isolated cells 

 eventually become 

 blood-corpuscles. They 

 collect dorsally during 

 the upward growth of 

 the primitive segments 

 (Fig. 45 B), and, as 

 they press somewhat 

 closely against one an- 

 other, they form (especially in the abdomen) a compact strand of 

 cells which prevents the junction of the primitive segments in the 

 dorsal middle line (Fig. 46, bl). Subsequently the mesoderm grows 

 between this strand and the ectoderm, and thus the two primitive 

 segments meet to form a partial dorsal mesentery. At a later period, 

 the walls of the primitive segments grow between the -yolk and this 

 strand of cells, and unite with one another below the latter (Figs. 46 

 and 47 A). This strand of cells has consequently become enclosed 

 by a layer of mesoderm having the form of a longitudinally-placed 

 tube, which is at first attached to the somatopleure above and the 

 splanchnopleure below. The tube soon loses its connection with its 

 parent mesoderm (Fig. 47 B), and we now find a continuous layer of 

 mesoderm (somatopleure) lining the ectoderm, while another layer 

 covers the yolk (splanchnopleure) ; between these two layers is the 

 body-cavity, in which the mesodermal tube now lies freely. This 

 tube is the heart, and, so far as can be judged, it is formed directly 

 from the walls of the primitive segments (Schimkewitsch, Locy. 

 Mokin (Fig. 47 A and B)). As a consequence of the development of 

 the heart, the primary continuity of the cell-elements of the 

 primitive segments becomes interrupted at this point (Fig. 47 B). 



