56 DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK. 



is, however, probable that, in the very first formation of the 

 heart, the cavity is single, being formed after the two ends of 

 the folded mesoblast have united (vide Ji z, fig. 10). In some 

 cases the two folds of the mesoblast appear not at first to 

 become completely joined in the middle line ; in this case the 

 cavity of the heart is still complete from side to side, but the 

 mesoblast-cells which form its muscular walls are deficient 

 above. By the process of absorption, as I said, a cavity is 

 produced in the thickened part of the mesoblast of the splanch- 

 nopleure, a cavity which is single in front, but becomes divided 

 further behind, where the folds of the mesoblast have not united, 

 into two cavities, to form the origin of the omphalomeseraic 

 veins. As the folding proceeds backwards the starting-point 

 of the omphalomeseraic veins is also pushed backwards, and 

 the cavities which were before separated become joined to- 

 gether. From its first formation the heart is lined internally 

 by an endothelium ; this is formed of flattened cells, spindle- 

 shaped in section. The exact manner of the origin of this 

 lining I have not been able to determine ; it is, however, probable 

 that some of the central mesoblast-cells are directly converted 

 into the cells of the endothelium. 



I have obtained no evidence enabling me to determine 

 whether Dr Klein is correct in stating that the cells of the 

 mesoblast in the interior of the heart become converted partly 

 into blood-corpuscles and partly into a cellular lining forming 

 the endothelium of the heart, in the same way that the blood- 

 vessels in the rest of the blastoderm are formed. But I should 

 be inclined to think that it is very probable certainly more 

 probable than that the cavity of the heart is formed by a pro- 

 cess of splitting taking place. Where I have used the word 

 "absorption" in speaking of the formation of the cavity of the 

 heart, I must be understood as implying that certain of the 

 interior cells become converted into the endothelium, while 

 others either form the plasma or become blood-corpuscles. 



The originally double formation of the hinder part of the 

 heart probably explains Dr Afanassiev's statement (Bulletin dc 

 rAcadcin. Iinpcrialc dc St Petersb., torn, xiii, pp. 321 335), that 

 he finds the endothelium of the heart originally dividing its 

 interior into two halves ; for when the partition of the mesoblast 



