92 BULLETIN OF THE ESSEX INSTITUTE. 



larly and is slender at the point of union. A point which 

 seems to me of considerable morphological importance is 

 that the symphysis is considerably behind the apices of 

 the trabecule, recalling in this embryonic stage the con- 

 ditions which are permanent in the Elasmobranchs and 

 lower Ganoids. 



Second STAGE. — Embryo, eleven mm. long (Fig. 3). 

 The parts described in the first stage have chondrified 

 rapidly and two new pairs of cranial elements, the otic 

 capsules and the two processes of the occipital arch, have 

 made their appearance. 



The occipital arch {ocp) is composed of two isolated 

 conical pieces of cartilage, the bases of which rest upon 

 the sides of the notochord a little behind the otic region. 

 From the notochord they curve outwards and upwards, half 

 encircling the medulla and terminating freely. The in- 

 completely developed otic capsules (o) are represented by 

 thin layers of cartilage, very irregular in outline, cover- 

 ing the lateral and portions of the dorsal and ventral sur- 

 faces of the auditory vesicles. Their anterior ends lie 

 immediately behind and above the bodies of the quad- 

 rates while their posterior ends are somewhat nearer the 

 median line, just in front of the occipital arch. The ven- 

 tral walls of the two capsules are in nearly the same hor- 

 izontal plane with the notochord and the parachordals. 

 There are no cartilaginous connections between the cap- 

 sules and the rest of the chondrocranium. 



The parachordals (j?) have fused with each other at the 

 apex of the notochord and now extend back along its sides 

 to about the middle of the otic region where each gives 

 off a lateral process which underlies the median portion of 

 the sacculus. Although the median wall of the otic cap- 

 sule is as yet but slightly developed, its future lino of 

 fusion with the parachordal is clearly indicated by a lon- 

 gitudinal marginal thickening of the latter. 



