42 CHARLES R. STOCKARD - 



of the intermediate cell mass has not been satisfactorily shown. 

 This question will be more fully considered in another section. 



In the embryos in which the red blood cells remain confined 

 within the median region throughout life these cells develop in 

 a normal manner and become completely differentiated into 

 typical ichthyoid erythrocytes and exist as such for some time. 

 Finally, however, for reasons at present impossible to state 

 but hkely associated in some way with an insufficient supply of 

 oxygen, these erythrocytes begin to degenerate and in old 

 embryos of sixteen to twenty days only a very few or in some 

 cases none are left in the large intermediate vessel. Mesenchymal 

 cells seem to wander into the mass of erythrocytes and may 

 take part in their destruction. 



Figure 41 is a section through the intermediate cell mass of a 

 sixteen-day old embiyo and presents this degenerate condition. 

 The erythrocytes are all small and necrotic and many mesenchy- 

 mal cells are scattered among them. 



As we shall see below, the power of existence of the erythro- 

 cytes is very much stronger in the blood islands where aeration 

 is no doubt considerably better than in the intermediate cell 

 mass. 



3. Blood isla7ids of the yolk-sac, their origin and development 



The question of origin of blood cells on the yolk-sac of the Tel- 

 eostian embryo has been a much debated topic. Almost all of 

 the earliest workers claimed that blood arose in the yolk-sac 

 islands of the bony fish just as in other meroblastic eggs. The 

 later workers, however, have denied this statement and hold that 

 the bony fish forms an exception to the rule, and is the only type 

 of meroblastic embryo in which blood cells do not occur in islands 

 on the yolk-sac. 



It has been frequently admitted by several recent workers that 

 certain wandering mesenchymal cells do migrate to the yolk- 

 sac from the embryo and there form isolated blood cells or 

 small cell groups, but that this blood formation is insignificant 

 in amount as compared with the great blood forming intermediate 

 cell mass. 



