338 Robert J. Terry. 



caudad, the site of the posterior commissure is indicated by a 

 slight projection of the roof into the ventricle and the presence 

 of a small elongate area of clear protoplasm (Comp. Hill, '99). 

 Between the epiphyseal arch and the site of the posterior commis- 

 sure, there appears a segment of the brain-roof not seen in the 

 older embryos as a separate region. Its relations identify it 

 as the pars intercalaris (fig. 14, PI). The segment of the roof 

 anterior to the epiphyseal arch is thicker than that which is 

 posterior; also it is relatively and absolutely thicker than the 

 corresponding part in older stages. It presents, as in the 5 mm. 

 embryo, a flexure in the sagittal plane, but exhibits no trace of 

 the subdivisions evident at that stage. 



Opsanus Larvae 15 mm., in Length 



« 



Epiphysis. In fig. 15, taken from a section a little to one side 

 of the median plane, the epiphysis is seen to be inclined well 

 forward over the paraphyseal region of the fore-brain. The epi- 

 dermis is now folded to form a deep, wide groove extending 

 transversely between the lines of the supraorbital pit organg'(fig. 

 15C,). Although standing in a position posterior to the level of 

 groove, the epiphysis is inclined forward so that its axis, if pro- 

 longed, would meet the groove. This topographical relation is 

 the only one which was observed between the two organs. The 

 small epiphyseal bud, noted in younger stages, is not present. 

 A mesenchymal layer, in which the bony cranial roof is to form, 

 now covers the great dorsal fontanelle of the chondrocranium. 

 Over a considerable area of this layer are attached strands of 

 connective tissue which radiate from the end of the epiphysis. 



Superior Commissure. Owing to the disappearance of the post- 

 velar arch, the superior commissure now lies between the velum 

 and the epiphyseal stalk. 



Posterior Commissure. The two divisions are even more sharply 

 limited toward each other than is the case in the earlier stages. 

 The mesenchymal septum is a well defined fold in a membrane 

 which, followed posteriorly, covers the mid-brain, and anteriorly 

 joins with the connective tissue over the diencephalon. The 



