REMOVAL OF NASAL PITS AMBLYSTOMA EMBRYOS 39 



the same time the mesenchyme interposed between the eye and 

 the brain is converted into a flat plate of cartilage, the crista 

 trabeculae, by chondrification, which, starting from the tabecula, 

 proceeds dorsally. In the early stages this plate does not extend 

 anteriorly beyond the caudal end of the olfactory sac. With 

 the formation of these cartilages, there appear two processes, 

 one a lateral process — the antorbital, and the other a median 

 process. The antorbital appearing first as a more or less straight 

 rod projecting outward, eventually becomes hooked anteriorly 

 about the posterior nares. 



The median process projects medially from the trabecula, just 

 in front of the anterior end of the telencephalon. It eventually 

 meets and fuses with its fellow from the opposite side. This 

 solid bar, whose caudal margin is later thinned out into the thin 

 floor of the anterior part of the cranium, becomes the ethmoidal 

 or anterior trabecular plate. 



With the advancing chondrification of the trabeculae is corre- 

 lated the formation of a cartilaginous plate underlying the nasal 

 sac, supporting it and Jacobson's organ. This solum nasi extends 

 under only the anterior two thirds of the olfactory organ. 



During all these earlier stages of development the nasal placode 

 is closely applied to the latero-ventral surface of the telencepha- 

 lon. As growth proceeds, the sac begins to push anteriorly and 

 at the same time laterally, leaving thus a clearly marked olfac- 

 tory nerve. In the mesenchyme which lies above this early 

 olfactory nerve there appears as a separate chondrification a small 

 rod of cartilage — the ethmoid column. This grows in an antero- 

 posterior direction, becoming fused posteriorly with the crista 

 trabeculae. Anteriorly, it grows toward the trabecula sending 

 a short process to meet it before attaining to its maximum length. 

 In front of this process is formed the narrow slit, the medial 

 incisure through which passes a branch of the nasalis internus. 

 Subsequently this slit is reduced to the foramen apicale. The 

 growth of the ethmoid column posteriorly to unite with the 

 crista and anteriorly to join the trabecula forms a bridge over 

 the olfactory nerve, which constitutes the roof of the fenestra 

 olfactoria (fig. 8/0). At the same time there appears the medial 



