HEAET IX PETROMYZOX. 233 



hypoblast, and in Ijonibinator (5), of both the hypoblast and the 

 mesobhist.'^ 



As to the origin of the mass of cells, marked (a;) in mv Figs. 3, 

 5, 8, and 10, which is destined to become the endothelium, I am 

 unfortunately unable to say anything definite, but I am inclined to 

 regard it as derived from the mesoblast. These cells much 

 resemble the mesoblastic elements : they are small, round, compact 

 bodies, free from yolk-granules. Such a cell can not be detected 

 in the hypoblastic layer, and there is found no trace of prolifera- 

 tion of cells in the ventral mtirgin of the enteric wall, which 

 presents a sharp C(3ntour throughout. On the contrary, the fact 

 observed by both Shipley and myself that cells are detached from the 

 free edges of the lateral plates of the mesoblast and fall into the space 

 between the epiblast and the hypoblast, speak in flxvour of the view 

 that the cell-mass may have been thus derived from the mesoblast. 



Shipley believes that these detached cells are destined partly to 

 Ibrm the ventral portion oî the mesoblast and partly to give rise to the 

 blood-corpuscles. Formerly I agreed with him in the first half of his 

 statement, but further study forces me to conclude that these cells 

 never have any share in the formation of the mesoblast, l)ut are solely 

 employed in building up the he:u't and the main trunks of the blood- 

 vessels."^ Their detachment and their accumulation in the ventral 

 median line are observed both in the anterior and the posterior 

 regions. Xow there may be some reason in thinking that in the 

 posterior region where the gastral mesoblast does not reach one half 

 of the distance to the ventral median line, addition of free cells to its 



^) I have not read his complete work on Petroaiyzon : Abhandhingen zur Entwicklungs- 

 geschichte der Thiere. 5. Hft. Die Entwicklungsgeschichte des Flussneunauges. Hamburg 

 und Leipzig, 18S8. 



-) It must be noticed that in the cardiac tube, when first formed, there is found no trace of 

 blood-corpuscles. 



