36 CHARLES R. STOCKARD 



be based only on embryos that have been carefully observed 

 throughout their existence. 



Figures 29 and 30 show sections through different parts of a 

 solid heart string from an embryo of sixteen days old. In 

 figure 29, the aortic end of the heart is shown to be almost a 

 solid mass, and only near the center of the figure is a slight 

 endothelial-like cavity or formation. 



Figure 30 is a cross-section through the long string-like por- 

 tion of this heart. It is seen to be completely solid, the central 

 portion or core consisting of periblastic material containing large 

 amorphous periblast nuclei, pb. Chromatophores have almost 

 ensheathed the structure and present in the figure a dense black 

 border. In one part of the section, however, a distinct endothe- 

 lial-like formation is shown surrounding the periblastic core, and 

 this heart again would seem to have sucked itself full of peri- 

 blastic material from the surface of the yolk. As stated, this 

 heart was from an embryo sixteen days old in which the 

 blood had never circulated, and it is quite evident that at the 

 time the embryo was fixed, it would have been impossible to 

 have had a circulation of blood through such a solid heart. 

 In this specimen, however, numerous blood islands on the "yolk- 

 sac and well formed blood cells in the intermediate cell mass 

 were to be seen. 



The endothelial lining of these hearts has certainly arisen in loco, 

 and has emphatically not grown into the heart from the yolk- 

 sac vessels since the heart is not connected with such vessels, 

 and further, no typical vessels are present on the anterior portion 

 of the yolk-sac. In all cases, the intra-embryonal vessels are 

 much better developed than the vessels of the yolk-sac. A gen- 

 eral survey of these embryos would quickly convince one that the 

 vessels within the embryo are in no case derived from ingrowths. 

 This fact is peculiarly emphasized in a study of bony fish embryos, 

 and is so convincing that it led Sobotta ('02) to develop a theory 

 of vascular outgrowth from intra-embryonic vessels in contrast 

 to the older parablast notion of His ('75), but I must agree 

 with Mollier ('06) in his view that both theories are equally 

 untenable. 



