244 H. D. Senior. 



Correlation of stages. 

 The embryo becomes sufficiently straight to yield a satisfactory 

 end-to-end measurement when it has about 51 somites; its length 

 is then 3.6 mm. From this time on the stages are designated by the 

 length in millimeters; it may be mentioned that the last somite 

 (59th or 60th) is formed at the stage of 5.2 mm. 



Mechanism of the circulation at different stages of development. 



Prior to the stage of 42 somites there has been no true circulation, 

 because the aorta opposite the first three somites, late in being 

 formed, is still wanting. At 42 somites the aorta is practically com- 

 plete as far back as the anus; posteriorly it bifurcates, and the two 

 vessels form a loop encircling the gut and join the subintestinal vein. 

 The blood plasma (for there are no corpuscles) flows from the 

 heart into the aorta, turns forward near the anus, into the sub- 

 intestinal vein, which, in turn, discharges it on to the posterior 

 pole of the yolk. The plasma flows ventral and lateral to the yolk 

 in a wide channel between the ectoderm and the yolk-periblast 

 and, following the contour of the yolk, enters the venous end of the 

 heart-tube. The plasma is not in contact with the periblast ventral 

 to the peritoneum, for it is excluded from this situation by the at- 

 tachment of the lateral margins of the latter to the yolk; as far as 

 can be determined the periblast elsewhere is bathed in plasma. The 

 caudal aorta and vein are as yet unformed, but a cord of cells ventral 

 to the notochord in the tail represents these vessels together with the 

 blood-anlage. The jugular and cardinal veins are not yet developed; 

 there is no liver. 



The jugular veins have reached completion at a stage of J^.Jf. mm., 

 and discharge their blood upon the yolk immediately posterior to 

 the vagus ganglia. It may be mentioned that the endothelial cells 

 of the jugular veins have been recognizable for some time prior to 

 the complete formation of the veins themselves, certainly as early 

 as the stage of 42 somites. The place of termination of the jugular 

 veins is a point of great interest, for here the ooelom is separated 

 into its pericardial and its peritoneal portions. The orifices of 

 discharge of the jugular veins are placed laterally to the lateral 



