102 BULLETIN OF THE ESSEX INSTITUTE. 



sular floors, continue forward into the trabecule. The 

 notochord projects forward some distance in front of the 

 most anterior point at which the parachordals touch its 

 sides. The otic capsules are widely separated and incom- 

 pletely formed. Their floors are partially attached to the 

 parachordals and the only traces of the median wall are 

 found around the inner mar<>:ins of its walls. A laro;e 

 fenestra ovalis, with no stapes, and a large foramen for 

 the exit of branches of the seventh nerve constitute the 

 only well defined apertures which are found in later stages. 



The trabecule extend forwards from the anterior ends 

 of the parachordals as long slender rods running parallel 

 for the posterior three-fourths of their length and then 

 bending inward at an angle of about thirty degrees 

 towards the median line, which they do not reach. There 

 are, as yet, no foramina for the second and third nerves 

 and only a slight trace of a trabecular crest to which the 

 ascending process of the quadrate is attached. 



The quadrate is wedge-shaped as seen either from the 

 side or in front, the point being directed ventrally. A 

 short otic process extends upward and backward from the 

 dorso-lateral angle of the wedge and fuses with the otic 

 capsule, while a longer ascending process extends up- 

 ward and forward from the dorso-median angle and fuses 

 with the dorsal end of the slightly developed trabecular 

 crest. 



Second stage. — Our second model is that of an em- 

 bryo twenty-four mm. in length. The occi})ital processes 

 are fused distally with the walls of the otic capsules and 

 continue along the dorso-median angles of the capsules 

 as the beginnings of the synotic tectum. The jugular 

 foramen is at this staize a dorso-ventrally elongated slit. 



In place of the distinctly outlined parachordals of the 

 previous stage we now have a continuous basilar plate 



