30 ILLINOIS BIOLOGICAL MONOGRAPHS [30 



The dorsal surface of the roof of the internasal septum is surmounted by a 

 pair of dermal ossifications (Figs. 3, 22) which lie on each side of the median 

 line of the head. Near the anterior end of the cranium they fuse to form a 

 single median plate. The anterior ventral surface of this plate is fused to the 

 underlying perichondrial ossification by osseous trabeculae; posteriorly there 

 is a layer of connective tissue between these two ossifications. The paired 

 posterior pieces of the dermal ossification interdigitate with the anterior ends 

 of the paired frontal ossifications above the posterior end of the internasal 

 septum. The dermal element is the beginning of the dermo-supraethmoid part 

 of the adult bone, and the perichondrial, the auto-supraethmoid part. 



In the 60 mm. stage these ossifications have covered the ethmoid cornua 

 and extend laterally above the anterior end of the nasal fossa. There is also 

 a perichondrial and a dermal ossification on the ventral surface of the ethmoid 

 cartilage, which, though not apparent in the 32 mm. stage, are well developed, 

 and fused with each other in the 60 mm. stage. Both dorsal and ventral 

 dermal elements extend anteriorly beyond the cartilaginous cornua and fuse 

 with each other, forming a sharp anterior cranial edge, notched medially. 

 A small pocket is left in the wall of the nasal fossa between the lateral edges 

 of these ossifications which persists in the adult. The rudiment of the premax- 

 illary bone is fused to the ventral surface of the ventral dermal ossification. 



A thin flat plate of osseous tissue, derived from the ossification of a mem- 

 brane, lies along the lateral margin of the dorsal part of the ectethmoid process 

 (Figs. 3, 4). It projects laterally above the anterior end of the orbit and the 

 orbital foramen, and is the dermal portion of the ectethmoid ossification. 

 The rest of the ectethmoid process has hardly begun to show signs of ossifica- 

 tion and yet this dermal part extends from the anterior end of the orbit to the. 

 palatal articular surface (Fig. 3). Perichondrial ossification has taken place 

 on the ectethmoid process around this articular surface (Fig. 4), but the surface 

 itself remains unossified. Thus the early ectethmoid bone has both auto 

 and dermal parts. 



Two lateral line bones, the nasal and the lacrimal (Fig. 3, 22), form a part 

 of the roof of the nasal fossa on each side. The nasal {na) is a long straight 

 ossified tube extending parallel to the supraethmoid and separated from it by 

 a narrow space. It contains the anterior end and opening of the supraorbital 

 latero-sensory canal. The process of the formation of such a bone as this has 

 been described by Piatt (1893) and Klaatsch (1895), so that it is unnecessary 

 to repeat it here. 



The lacrimal lies near the ventro-lateral margin of the roof of the nasal 

 fossa and contains the anterior end of the suborbital latero-sensory canal. Its 

 triangular outline is due to the division of the canal into two tubes for com- 

 munication with the exterior. Ventro-posteriorly it articulates with the 

 anterior ossicle of the infraorbital series (Fig. 3) and anteriorly it is fastened by 

 connective tissue to the dorsal surface of the palatine ossification {pal). Be- 



