710 DEVELOPMENT OF THE COMMON PHALANGER, 



numerous round the posterior half of the canal than in the 

 anterior half. In the antero-lateral horn the group of neuro- 

 blasts from which the anterior roots are spi-inging is compara- 

 tively small. Both anterior and posterior white columns are 

 as yet small. 



Heart and vessels. — The two auricles communicate freely in the 

 upper part below the margin of the septum superius, but are 

 divided below by the septum intermedium. The septum inferius 

 does not reach the auriculo-ventricular ostium ; the ventricles 

 being thus in free inter-communication. The sinus venosus is 

 comparatively small. The truncus arteriosus is undivided to the 

 level of the upper third of the auricles. The condition of the 

 main blood vessels I have recently elsewhere described.* It may 

 here be briefly noted that the remains of the right aorta and of 

 the right ductus arteriosus Botalli can still be traced, and that 

 the right pulmonary artery comes off very distinctly from the 

 right fifth (6th V) arterial arch, the left pulmonary artery coming off 

 as distinctly from the left arch. The condition of the umbilical 

 veins is very interesting in that, though the embryo is in some 

 respects more advanced than a six weeks' human embryo, there are 

 still two umbilical veins present, the right being about as large 

 as the left. 



Lwxja. — The degree of ramification of the bronchial tubes is 

 about equal to that of a seven weeks' human foetus, though the pro- 

 portion of mesoblastic tissue into which the sprouting is taking 

 place is considerably less. 



Alimentary atrial, &€. — The oesophagus is small and for a shorr, 

 distance in the neighbourhood uf the aortic arcli the lumen is lost. 

 In the iovver part tlie lumea is exceedingly small. The stomach 

 and duodHnnm nre well developed, but below the duodenum the 

 lumen of the canal steadily diminishes, and for some distance iu 

 the neighbourhood of the vitelline duct, traces of which are still 

 apparent, the lumen is absent. The canal continues very small 



* R. Broom. "On the arterial arches and great veins in the fceta^ 

 Marsupial." Journ. Anat. and Phys. Vol. xxxii. p. 477, April, 1898. 



