162 



deficiency, his admirable account of the organ agrees exactly with the 

 condition found in my specimens. 



From Dr. A. Hill's account of the brain one would judge that 

 his material must have been in anything but a good state of preser- 

 vation. His whole account is highly inaccurate and misleading. For 

 instance, speaking of the olfactory bulb, he says : "it is absolutely free 

 from the cerebral hemisphere, the rounded neck of the crus . . . 

 being crossed by the large anterior cerebral artery, . . . The bulb is 

 cupped on its under surface" ^). It is hard to understand how he 

 has distorted the facts to fit in with this description, as every state- 

 ment is absolutely erroneous. Neither is the bulb free from the hemi- 

 sphere, nor is the cupping on the inferior, but on the upper part of 

 the lateral surface. 



[The "anterior cerebral artery" of Hill is a cerebral vein, which 

 (in the absence of a superior longitudinal sinus) is very large and 

 constitutes the main efferent channel for the blood of the cranial 

 cavity.] 



Professor Wilson very gene ously placed at my disposal a com- 

 plete series of coronal sections of the head of a foetal Platypus 2), 

 the comparison of which with the adult proved very instructive. In 

 the foetus the organ of Jacobson forms a sac 1,35 mm long. In 

 general appearance it presents a considerable resemblance to the 

 adult condition of the organ in Tropidonotus natrix, described 

 by Dr. J. Beard ^). In transverse section it presents a roughly cres- 

 centic outline (Fig. 1 J"), the outer wall (Fig. 2 I), which is composed 

 of low cubical epithelium, being bulged in by a slight projection of 

 the cartilage of Jacobson (Fig. 1 t). 



This slight projection is the rudiment of the large turbinate fold 

 (Fig. 3 t) found in the adult and which Symington has accurately de- 

 scribed. Exactly midway between its anterior and posterior extremities 

 the sac communicates (d) with the naso-palatine canal of Stenson (Sd). 

 The mesial wall (Fig. 2 m) of the sac differs markedly from the outer 

 wall [1). It consists of a dense mass of cells with deeply stained 

 nuclei. Many of these cells have a distinctly columnar form and 

 many of them are to be recognised as neuroblasts connected with 

 nerve fibres in the deeper layer. In the adult this mesial wall 



1) Philosophical Transactions, 1893, B p. 374. 



2) The external measurements of this foetus have been given in 

 detail in the Proc. Linn. Society of New South Wales, Vol. IX, 2nd Series, 

 p. 682. — The cerebrum measures 5,15 mm in length. 



3) Zoologische Jahrbücher, Bd. 3, Anat. und Ontogenie, p. 769. 



