580 DR. J. SYMINGTON ON THE [Nov. 17, 



columnar epithelial cells next the free surface such as are generally 

 found. It is possible that these have been detached. 



In the Guinea-pig, Dog, and Rabbit Dr. Klein found the organ of 

 Jacobson to be flattened from side to side with the outer wall 

 pushed slightly inwards, so that on transverse vertical section the 

 organ was kidney-shaped. The outer wall was covered with ciliated 

 columnar epithelium, while the inner wall possessed, in addition to 

 ordinary columnar cells, special sensory cells. The outer wall of the 

 organ in these mammals evidently corresponds to that covering the 

 turbinated process in the Ornithorhynchus, while the inner wall is 

 represented by the structures lining the general cartilaginous capsule. 



In all mammals so far as at present investigated, except the 

 Monotremes, the cavity of Jacobson's organ communicates anteriorly 

 with the nasal chamber or with Stenson's duct, and this opening is 

 anterior to the cavity of the organ. We have already seen that this 

 is not the case in the Ornithorhynchus, where the cavity extends 

 forwards as well as backwards from its opening into Stenson's duct. 

 In Lizards the duct from Jacobson's organ passes backwards and 

 downwards to open into the cavity of the mouth. 



The cavity of the organ of Jacobson in the Ornithorhynchus is 

 about 5 to 6 mm. in length. Both extremities end blindly, the 

 posterior one just in front of the hinder eud of the dumb-bell-shaped 

 bone. 



The ridge on the inner wall of the nose caused by Jacobson's 

 organ is prolonged backwards nearly 3 cm. beyond the termination 

 of the organ proper. The ridge here consists of glandular tissue 

 and bundles of olfactory nerve-fibres, the nerves being internal to 

 the glandular tissue (see fig. 3, Plate XLIIL). 



The nasal cavity is lined by stratified squamous epithelium until 

 near the posterior end of Jacobson's organ, where it becomes gradu- 

 ally replaced by columnar epithelium. 



A transverse vertical section of the nose (see fig. 3, Plate XLIII.) 

 about 1-5 cm. behind the nostrils shows that the walls of the nose 

 are still mainly cartilaginous ; the floor, however, contains the 

 palatine process of the superior maxillary bone, and the lower part 

 of the nasal septum the vomer. 



The Bumb-bell-shaped Bone. 



Since 1879, when Professor Albrecht (3 a) published his first paper 

 dealing with the ossification of the inter-maxillary bone, there has 

 been a vigorous controversy as to whether this bone is normally 

 developed in man and the higher mammals from one or two centres. 

 The embryological evidence in favour of two centres appears to me 

 to be unsatisfactory, the careful observations of T. KoUiker (16) 

 and Schwink (17) being strongly in favour of its formation from a 

 single centre. 



Albrecht's (3) views as to the morphology of the dumb-bell- 

 shaped bone in the Ornithorhynchus have, ' however, been pretty 

 generally accepted. He directed attention to the fact, previously 

 noticed by Rudolphi, Meckel (1), and Owen (2), and since confirmed 



