OPMENT OF COXNB KSTANCE AND BLOOD. 





If one at this time examines a Ma-oderm which has been removed 

 from the yolk. the /one in which tin- formation of blood tak- > placo 

 appear- tl eked with more or less intensely coW-d l>lood red .-.pots, 

 some of which arc roundish, others elongated, and others 1. ranched. 

 The spots are known as the blood-points or blood-islands of the M 

 derm (ii;. 114). From these formative areas the superficial cells 

 now detach themselves and enter the blood fluid as the i><>lated red 

 blood corpuscles. Here, as well a~> in the blood-islands, they multiply 

 1'V means of cell-division, during which the nucleus is metamorpi 

 into the well-known spindle-figure. 



AJ REMAK first showed, < ! ;,-'/^ions of blood-cells are to he obs< i 

 in tlie Chick in great numbers up to the sixth day of incubation, 

 whereas they later become more rare, and then wholly disappear. 

 Also in the case of Mammals and of Man (FoL) the first embryonic 



61 



Fig. 115. Cross section through a portion of the vascular area, after DISSE. 



at, Outer, ik, inner germ-layer ; mk\ parietal, mk*, vi.-H-t-r.il lamella ff the middle germ-layer; 

 lh, extra-erabryouic body-cavity ; gw, wall of blood-vessel formed of endotheliutn ; bl, blood- 

 cells ; g, vessels. 



blood-corpuscles, which are at this time provided a* in the other Verte- 

 brates with a genuine cell-nucleus, possess the power of division. 



In proportion as blood-corpuscles .still further detach themselves 

 from the blood-points, the latter become smaller and smaller, and 

 finally disappear altogether ; but the vessels without exception 

 contain, instead of a clear fluid, red blood with abundant formed 

 elements (fig. 115 bl). 



Subsequently there occur changes in iheSubstanzinseln which lead 

 to the formation of embryonic connective substance. The germinal 

 cells, at first .spheroidal, separate farther from one another, at the 

 same time secreting a homogeneous inter-cellular substance; thev 

 become stellate (fig. 116 sp), and send out processes by means of 

 which they are united into a network, which stretches all through 

 the gelatinous secretion; other cells apply them-elv.s to the endo- 

 thelial tubes of the vessels. 



