96 FRANKLIN P. REAGAN 



be observed through each translucent otocyst. Within the next 

 three days a single median hematopoetic area was observed in 

 the mandible. These conditions continued for some time. Fig- 

 ure 58 shows the conditions on the twenty-fifth day. Blood cells 

 were observed apparently scattered on the floor of the pericar- 

 dium. Later sectioning of this embiyo indicated that the eryth- 

 rocytes on the floor of the pericardium had formed locally be- 

 neath the mesothelium. 



It sometimes happens that embryos develop without forming 

 a heart; sometimes there is little, if any embryonic body present. 

 In such cases the entire yolk-surface may be covered with blood 

 cells, diffusely scattered or in groups; such a condition may ob- 

 tain when there is practically no endothelial tissue present. 

 Such conditions furnish a positive disproval of the claim that blood 

 cells never form on the anterior surface of the yolk if there has 

 been no circulation. Figure 59 shows a condition in which a 

 small portion of the body axis abuts on a coelomic cavity which 

 appears to be a pericardial cavity. No heart was ever formed in 

 this embryo, yet no region of the yolk was free from blood cells. 

 In figure 60 a condition is represented in which no embryonic 

 body ever developed. No particular region of the yolk, even 

 from the earliest stages following chemical treatment, seemed es- 

 pecially entitled to be designated as embryonic body. The 

 blood cells were somewhat scattered, but in some cases they were 

 densely crowded. In the center of the region towards the ob- 

 server, a blood-mass is surrounded by endothelium. 



A very unusual condition was observed in the embryo from 

 which figures 69 and 70 were made. At its four-cell stage this 

 embryo was treated for twenty-four hours with a 3.5 per cent 

 solution of alcohol. On the sixth day a bright red blood-lacuna 

 was observed on the antero-ventral surface of the yolk. No 

 other erythrocytes could be seen on the yolk or in the body axis 

 at this time, though mesenchyme cells in abundance were migrat- 

 ing on the yolk. There was no very extensive system of endo- 

 thelial tubes. The heart was apparently normal, but its venous 

 end had not connected with any yolk-sac vessel. On the tenth 

 day the blood-lacuna on the anterior yolk, somewhat elongated. 



