718 DEVELOPMENT OF THE COMMON PHALANGER, 



tions where enlarged hair^ (Hactile hairs) are met with in the 

 later mammary fcetusea. The dental lamina can be traced back 

 to the plane of the eye, and the dental germs of the anterior teeth 

 are well differentiated. 



Skihton. — The cartilaginous skeleton is almost completely 

 formed. The pelvis and the skeleton of the posterior extremities 

 are fairly well chondrified, and the vertebral centra can be traced 

 for a considerable distance along the tail. The sternum is 

 developed, but at its lower half the two sides are not yet com- 

 pletely united. The coracoid is well developed and articulates 

 with the sternum, and the clavicle is well ossified. The chondro- 

 cranium is complete, and the following bones are well ossified — 

 premaxillaries, maxillaries, palatines, and mandibles. 



Nervous system. — The brain and spinal cord are not very 

 satisfactorily preserved in this embryo. In the cord the grey 

 columns are still further developed, giving the cord on section a 

 more rounded appearance. The central canal still extends to the 

 posterior part, and there is as yet no trace of a posterior fissure. 



Sense organs. — The nose is well formed, and as the palate is 

 now closed, the appearance differs but little from that of the 

 adult, save that the turbinals are as yet rudimentary. The eye is 

 still but imperfectly developed. The retina is a fairly thick 

 laj^er, and is not yet differentiated in its structure- The 

 pigmented layer is only pigmented in its anterior two-thirds. 

 The optic stalk still has a distinct lumen. The internal ear is 

 fairly well developed — the saccule and utricle are quite distinct, 

 the semicircular canals well formed, and the cochlea of large 

 size, though still but slightly curved. 



fJeart and vessels. — ^The two ventricles are now completely 

 divided, and the aortic bulb is likewise completely subdivided. 

 The pulmonary arteries arise from the pulmonary in a common 

 though exceedingly short stem. The apparent difference in the 

 mode of origin of the arteries from that in the earlier embryos is 

 due to the points of origin becoming approximated with the 

 lengthening of the embryo and their ultimately coinciding. The 

 left umbilical vein now lies almost in the middle line, and the 



