142 Messrs. W. K. Brooks a7id G. Lefevre on 



There is no reason for believing that these cells are not 

 true mesodermal cells, for in the earlj stages of development 

 the similarity between many of the free cells of the blood 

 and those which make up this mass is perfectly apparent. 

 There is certainly not the slightest evidence that the wall of 

 the inner vesicle evaginates or its cells proliferate at this 

 point, the line of demarcation between the two structures 

 being distinct and unbroken throughout. 



With the displacement of the vesicle the rudiment, which 

 is firmly attached to the wall of the latter, is passively carried 

 down to the ventral side ; but long before it has reached its 

 definitive position it has become thicker and more compact, 

 and a cavity has appeared in its centre, around which the 

 cells arrange themselves into a one-layered epithelium to form 

 an elongated closed sac. 



When the change in position of the vesicle is completed 

 the pericardial sac loses its attachment to the wall, and a 

 longitudinal folding-in of the dorsal side takes place in the 

 usual way to form the heart. 



The Dorsal Tube and Ganglion. 



Very soon after the cells which make up the rudiment of 

 the pericardium have made their appearance a similar collec- 

 tion of cells is found on the outer surface of the inner vesicle 

 somewhat to the left of the median dorsal line in the anterior 

 end of the bud. These cells, which are at first very loosely 

 grouped together, form an irregular elongated mass, the 

 rudiment of the dorsal tube ; and so gradual is the transition 

 from the surrounding cells of the blood to those of the rudi- 

 ment, that at this early stage it is impossible to say where 

 the former end and the latter begin. 



As in the case of the pericardium, there is every reason to 

 believe that the dorsal tube is likewise formed of true meso- 

 dermal cells. The line of separation between the rudiment 

 and the wall of the vesicle is perfectly distinct and clearly 

 marked, and there is no evidence whatever of proliferation 

 of endodermal cells at any point, while during the early 

 stages the cells which are to give rise to the tube and 

 the free amoeboid cells of the blood are identical in size and 

 appearance. 



The rudiment gradually increases in size, and the cells 

 composing it become closely packed together to form a solid 

 elongated mass, firmly adhering to the wall of the vesicle. 

 A lumen appears in the centre, around which the cells arrange 



