254 FRANKLIN P. MALL 



In my collection there are two other embryos slightly more 

 advanced in development than the one just described which bear 

 upon the exact origin of the atria from the heart tube. They are 

 Nos. 486 (4 mm. long) and 470 (4 mm. long). Neither of these 

 specimens has been studied carefully as a whole, so the number of 

 myotomes in each can not be given. Nor have the measurements 

 been corrected by the drawings and the sections. 



In No. 486 the single atrium as described above (No. 164) is more 

 pronounced, is dilated and filled with blood, v while the form of the 

 endothelial tube is much the same. However, the atrium is 

 sharply separated from the sinus venosus and there are a few 

 fibrillae between the endothelial and muscular walls. In the left 

 ventricle the endothelial and the muscular layers are just begin- 

 ning to interlock to form the first trabeculae. 



Embryo No. 470 shows the heart more advanced than in No. 

 486. The atrium has become double, that is there are two atria. 

 The endothelial tube in it is distended and its separation from the 

 sinus venosus is still pronounced. The trabecular formation in 

 the left ventricle is somewhat more pronounced than before. 



From now on the changes in the heart take place very rapidly, 

 as the general form of the embryo also changes rapidly. The head 

 is bent upon the body which is well curved upon itself with pro- 

 nounced limb buds. In the next embryo, No. 239 (4| mm. long), 

 the subdivisions of the heart are sharply defined and in the follow- 

 ing stage, No. 463 (3.9 mm.), the preliminary subdivisions are 

 complete (fig. 4). During this time the embryo curls upon itself 

 and the limb buds are formed. In embryo 239 the two atria are 

 very pronounced, the right communicating with the sinus venosus. 

 The atrial canal is sharply defined, first as a constriction and 

 secondly by a great increase of the fibrillar mass between the 

 endothelial and muscular walls. The trabecular system is well 

 formed in the left ventricle and has extended into the bulbus, 

 that is into the right ventricle. In this specimen it is clear that the 

 course of the circulation is from the right to the left atrium, then 

 first to the left ventricle after which it enters the right. The 

 right atrium still lies in the notch between the left ventricle and 

 the bulb on the posterior side of the heart. Only a little later 



