ORIGIN OF VASCULAR TISSUES 111 



jecting diagonally forward and upward in the median plane from 

 the postero-dorsal portion of the yolk is a slender group of eryth- 

 rocytes, the nature of which has not yet been determined, as the 

 embryo has not yet been sectioned. It is certain that these 

 cells are not a part of the intermediate cell-mass as such. It 

 will be noted that the dorsal portion of the trunk contains pig- 

 ment. The clear region ventral to this is filled with a clear fluid. 

 The pectoral fins are well developed. The mesenchjrme anterior 

 to the pectoral fin is hematopoetic. On the posterior yolk are 

 a few large blood-lacunae. Some smaller ones are found near 

 the diminutive pericardium. The region between these two 

 blood-bearing areas of the yolk does not contain endothelial 

 tubes. The intersomitic grooves are distinguishable in this em- 

 bry«; such is rarely if ever the case in chemically treated 

 embryos. 



Rather unusual conditions were obtained in another cardiec- 

 tomy (fig. 85). Before a pericardial cavity had been fonned, 

 and before heart-formation had taken place a very fine needle 

 was introduced ventral to the head-tissue. Not only was the 

 future heart-tissue destroyed, but the needle was then manipu- 

 lated so as to sever the body axis in the mid-brain region. The 

 needle did not puncture the body- wall ectoderm. A large spheri- 

 cal oedematous space probably corresponding to the pericardium 

 has formed; instead of forming anterior and ventral to the head- 

 fragment it has formed behind the latter, so that the head frag- 

 ment has been pushed considerably forward. The diameter of 

 the isolated cephalic tissue has increased very little beyond that 

 at the time of operation, whereas the main part of the body has 

 grown a great deal. The otocj^sts were partly destroyed, and 

 the remains of them have probably moved forward somewhat. 

 There was no heart-formation. Erythrocytes were observed 

 ventral to the otocysts and anterior to the pectoral fins. An 

 area probably indicating the point of emergence of the ducts of 

 Cuvier from the embryonic body contains erythrocytes. The 

 intermediate cell-mass is hematopoetic. The yolk-sac con- 

 tains numerous endothelial vesicles and pigment cells, but no 

 erythrocytes were detected on it in the living condition. 



