716 DEVELOPMENT OF THE COMMON PHALANGER, 



NervouH system. — The brain and spinal cord do not differ 

 greatly from those in Embryo C. The central canal of the cord 

 is still of great size, and occupies ^ of the antero-posterior 

 diameter of the cord. It is considerably dilated at its posterior 

 end, so that the canal on section is club-shaped. Cellular 

 elements (presumably spongioblasts) are very abundant round the 

 posterior part of the canal. The anterior cornu of grey matter is 

 very much larger than the posterior. 



Sense organs. — The nasal cavity is now well developed, though 

 still communicating freely with the mouth. Jacobsou's organ is 

 likewise well formed and rests on the now chondrified paraseptal 

 cartilage. The eye has not advanced much from the condition in 

 embryo C. There is still a fairly wide lumen in the optic stalk, 

 but the lumen of the lens is appreciably smaller. In the ear the 

 ^accule and utricle are now to some extent differentiated from one 

 another. The semicircular canals are well formed. 



Heart and vessels. — The division of the ventricle is not yet 

 quite complete, but the aortic liulb is considerably further sub- 

 divided than in Embryo C. The chief l^lood vessels, with the 

 exception of those belonging to the distinctively fcetal circulation, 

 agree in the main closely with the adult condition. The right 

 umbilical vein is still present, but it no longer opens into the 

 liver, and merely serves to convey some of the blood from the 

 anterior abdominal wall to the sinus along the jiosterior border of 

 the umbilicus. 



Lungs. — The bronchi are now well subdivided, and the lungs 

 are of fair size. The two main bronchi haA'e the mucous membrane 

 arranged in longitudinal folds. 



Alimentary canal. — The oesoj^hagus has a distinct lumen 

 throughout its whole extent. The stomach, duodenum, and upper 

 part of the small intestine are well developed, and in this jaart of 

 the intestine the mucous membrane is very much folded. In the 

 lower half of the small intestine the lumen is comparatively small, 

 and the mucous membrane is free from folds. There is still a 

 trace of the vitelline duct, and in the neighbourhood of the duct 

 the intestine is slightly more dilated. The c^cum, though 



