DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 



183 



If one at this time examines a blastoderm which has been removed 

 from the yolk, the zone in which the formation of blood takes place 

 appears flecked with more or less intensely colored blood-red spots, 

 some of which are roundish, others elongated, and others branched. 

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

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

 now detach themselves and enter the blood-fluid as the isolated red 

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

 by means of cell-division, during which the nucleus is metamorphosed 

 into the well-known spindle-figure. 



As HEMAK first showed, divisions of blood-cells are to be observed 

 in the Chick in great numbers up to the sixth clay of incubation, 

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

 Also in the case of Mammals nd <;f lfn (For.) the first embryonic 



a, 



ak 



ink 1 



mk* 



bl 



aw 



ik. 



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



ok, Outer, ik, inner germ-layer ; ;,ik\ parietal, //</.'', visceral lamella of the middle germ-layer ; 

 lh t extra-embryonic body-cavity ; git>, wall of blood-vessel formed of endothelium ; bl, blood- 

 cells ; g, vessels. 



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

 brates with a genuine cell-nucleus, 2)ossess 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 then 

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

 elements (fig. 115 bl). 



Subsequently there occur changes in the Substanzinseln 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 ; they 

 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 themselves to the endo- 

 thelial tubes of the vessels. 



