94 BULLETIN OF THE ESSEX INSTITUTE. 



tiou just mentioned, the lateral margins of the parachord- 

 als are now everywhere continuous with the floors of the 

 otic capsules. 



The otic capsule (o) now presents the typical capsular 

 form, covering by far the greater part of the surface of 

 the auditory vesicle. The median wall is beginning to 

 form along the dorsal margin and at the two ends of the 

 capsule, leaving two unchondrified spaces in the middle 

 and ventral region ; a large anterior one through which 

 pass the seventh and eighth nerves and the endo- and 

 perilymphatic ducts, and a smaller fontanelle lying pos- 

 tero-dorsally to the first. In the floors of the capsules 

 are the foramina for the exit of the branches of the seventh 

 nerve (vii) . Upon the left side the two branches of this 

 nerve pass through the two ends of a transversely elon- 

 gated foramen, while upon the right side they pass 

 through two separate foramina formed by the fusion of 

 the middle portions of the anterior and posterior walls of 

 the originally elongated foramen. Behind the foramina 

 for the seventh nerve and somewhat farther laterally is the 

 large fenestra ovalis (fo). The stapes (s) which arises 

 from a separate centre of chondrification occupies the 

 anterior end of the fenestra. 



The only noteworthy changes in the trabeculoe are a 

 general strengthening and an increase in the size of the 

 cornua (c). The quadrate has become more solid 

 throughout all its parts except the processes which were 

 at the base of the balancer in the preceding stage. They 

 have disappeared together with the balancer, leaving a 

 single conical process in the place they formerly occupied. 

 The otic process (op) extends considerably farther back- 

 wards than before. 



Fourth stage. — Larva thirty-nine mm. long. Gills 

 not yet atr(yphied (Figs. 8-11). The ventral portion of 



