230 S- HATTA; FORMATION OF THE 



ïhe ventral free edges of the mesoblast almost meet m the 

 ventral median line. Fiçf. 8 is the fifth section anterior to Fig". 9. 

 The first pair of the pronephric tubules are sliced (pn. t.); here the 

 mesenchymatous cells are still disjoined although the splanchnic 

 plates embrace them from right and left (x. and in. sp.). The re- 

 semblance between this and Fig. 3 is very striking and we are, I 

 believe, justified in concluding that the mass (x.) in Fig. 3 is destined 

 to form the endothelium. Posteriorlv to the section sho\vn in Fie. 9' 

 the mesenchymatous cells are either reticulated or aofojreofated , but are 

 not epithelially arranged. Fig. 10 represents the third section posterior 

 to that in Fig. 9 ; the mesenchymatous cells will 1)6 seen to be 

 reticulated (.t.). 



The heart in this stao;e, therefore, shows alreadv a tubular 

 structure in its middle portion while it is onlv an ao-oreg-ated mass of 

 cells both anteriorly and posteriorly. It is enclosed right and left by 

 the thickened splanchnic plates, and is sus[)ended to the body-wall and 

 the wall of the enteric canal. 



Fig. 11 represents a transverse section through the portion corres- 

 ponding to that in Fig. 9 of an embryo a little more advanced. 

 The second pair of pronephric funnels are cut througli (/;//. t.); the 

 endothelium is much thinned (ciul). The cardiac tulie has now 



acquired a lumen through its whole length, from the tniucus 



arteriosus to the sinus vcnosus. It shows almost the same features 

 everywhere. The dorsal and the ventral mesenteries or the mesocardium 

 posteriiis and the vi. aiiterius still suspend the heart in its entire length 

 (luc. p. and mc. a.). 



One step more (in the stage coinciding approximately with that 

 represented in Fig. 1 G): these mesocardial «uspensors begin to atrophy, 

 the process beginning at the middle section and extending gradually 

 both anteriorly and posteriorly, until finally the cardiac tube becomes 



