50 CHARLES R. STOCKARD 



reptiles as are the mammals, in fact, the connection between 

 the reptiles and birds is even closer. Yet the degenerating 

 nuclei of the reptilian blood corpuscle is able to maintain itself 

 in the corpuscles of the bird, although it is according to Minot, 

 so far gone as to degenerate entirely in the corpuscles of the 

 mammal. Such classifications are extremely misleading as they 

 convey to the mind the impression that there is a continuous 

 developmental or evolutionary chain of events illustrated in 

 the blood cells of the different vertebrate groups and actually 

 repeated in the development of the blood in the mammals. 

 The "biogenetic law" is scarcely vigorous enough at present to 

 be submitted to such a strain. 



Finally in considering these yolk-sac blood-corpuscles, one 

 must mention the possibility of origin from the yolk periblast 

 or endoderm. It is often stated even in modern text-books 

 and contributions that blood-cells may arise from endoderm and 

 that the primary blood forming layer was actually the endoderm. 

 It is very positively certain that none of the blood cells on the 

 yolk islands of the fish arise from the periblast, but all are de- 

 rived from wandering mesenchymal cells. The sharp distinc- 

 tion between endoderm and mesoderm is not a thing of any great 

 or definite importance, since everyone recognizes the primary 

 association and origin of mesoderm from the endoderm and the 

 ectoderm. When the mesoderm is once formed, however, it con- 

 tains within itself a blood forming anlage. It must be further 

 remembered by speculators on the phylogeny of the vertebrate 



Fig. 40 A highly magnified section through a yolk-sac vessel in a normal 

 embryo of seven days; Et, the vascular endothelium with chromatophores along 

 it. The large beautifully developed erythrocytes are seen in the lumen. 



Fig. 41 An equally magnified section through the intermediate cell mass 

 in an embryo without a circulation when sixteen days old; Embryo 413. This is 

 the only intraembryonic blood present, the vascular endothelium, which prob- 

 ably at one time surrounded the erythrocytes, has broken down and mesenchy- 

 mal cells, Men, are now intermixed with the small degenerate erythrocytes, Ery, 

 which should be compared with the normal ones in figure 40. 



Fig. 42 Shows erythrocytes in a yolk-sac vessel also in Embryo 413, at the 

 same magnification as in the two preceding figures. These erythrocytes are in a 

 better condition than the intra-embryonic ones, yet they are very degenerate 

 as compared with those of figure 40; all, however, still contain haemoglobin. 



