T*rry — The Nasal Skeleton of Amblystoma punctatum. 103 



tissue. It makes the medial wall of the nasal capsule m 

 its anterior part; ventrally it is still separated from the ex- 

 tremity of the trabecula by the notch (Inc. med.) described 

 for the 17mm. larva. 



The Lamina cribrosa (Winslow), (L.), is a broad plate of 

 cartilage, connected medially with the dorsal arch over the 

 olfactory window, and bent outward to cover the caudal part 

 of the main nasal sac above and at the side. It occupies 

 part of the perirhinal membrane, and while it consists of but 

 one piece it is fenestrated and irregular along its margins. 

 The pointed extremity of the Lamina is just dorsad of Jacob- 

 son's organ; a wide gap separates its caudal edge from the 

 antorbital process. 



The anterior trabecular plate (Ant. tr. pi.) is no longer in 

 a frontal plane but is inclined cephalo-dorsad so that it now 

 enters into both the floor and cephalic wall of the cranium. 

 The ethmoidal bridge, window and anterior trabecular plate 

 form a curved partition in front of and below the olfactory 

 bulbs. 



Stage IV. 

 Amblystoma, 27-30mm. Fig. 4. 



The nasal sac of this larva is only in its posterior half 

 alongside of the brain. Its contour is less regular than in 

 preceding stages, due mainly to the increase in the size of the 

 organ of Jacobson and to the further separation of this part 

 from the main-sac. The latter presents a great thickening 

 of its dorso-lateral wall appearing on the surface as a 

 rounded eminence sharply marked off from the middle lateral 

 region by a groove. The floor of this sulcus had already, in 

 the 21-24 mm. larva, lifted itself into a convexity and in the 

 present stage a small diverticulum of the main nasal cavity 

 bulges the floor into a little eminence (N. L. P. ) which 

 connects with the end of the Nasolacrimal Duct (D. nl). 



Jacobson' s organ lies ventrad of the lateral region of 

 the main-sac and has pushed its way mesad between the lat- 

 ter and the trabecular horn. Into its medial side opens the 



