BY R. BROOM. 713 



resting on the base of the nasal septum. Towards the posterior 

 of the palate plates the palato-pterygoid bar can be detected as a 

 narrow semi-cartilaginous rod. Meckel's cartilage is very well 

 developed, and the hyoid arch fairly well, as is also the thyroid. 

 The vertebral centra and arches are more or less chondrified 

 almost to the root of the tail. The ribs are also developed, with 

 the sternum, of which latter, however, the two halves are still 

 considerably apart. The scapula is well chondrified, and the 

 coracoid partly. The clavicle is very distinctly ossified. No 

 chondrification has as yet taken place in the pelvic arch. 



Nervous system. — The brain, on the whole, agrees roughly in its 

 degree of development \\\t\\ that of a sheep embryo of 15 mm., 

 though in one or two points the development is more delayed in 

 the marsupial. The hypophysis is quite unconnected with the 

 oral cavity. The spinal cord closely resembles that of Embryo A, 

 though the cellular elements, especially in the anterior half of tlie 

 cord, are considerably more developed. 



Sense orgayis.— With the exception of its communicating freely 

 with the mouth, the nose agrees essentially with the adult con- 

 dition. Jacobson's organ is already developed as a tubular 

 epithelial diverticulum. The eye corresponds roughly in its 

 degree of development to that of a human foetus of five weeks (His). 

 The optic stalk has still a faily wide lumen. The inner wall of 

 the optic cup is fairly thick, and in the outer there is some degree 

 of pigmentation. The lens is relatively considerably smaller 

 than in the corresponding stage of the rabbit or sheep ; and has' 

 a fairly large cavity. The condition of the internal ear agrees 

 pretty closely with that of the human foetus of five weeks. The 

 cochlea is a moderately long and fairly straight tube. The 

 saccule and utricle are still undifferentiated ; while the semi- 

 circular canals are all present as independent canals. The 

 recessus lab3^rinthi is present as an elongated somewhat clulj- 

 shaped epithelial process. The taste buds are very distinctly seen 

 on the tongue. 



Heart and vessels. — The heart is not developed much in 

 advance of the condition in Embryo A. The septum inferius 

 47 



