34 MR. W. K. PARKER ON THE STRUCTURE AND 



A remnant of the notocliord (nc) still pei'sists ; it lies on a thin junctional tract of 

 the investing mass through wliich it is very visible. 



The frontoparietals (f.J).) are but little changed, and are not yet segmented. The 

 parasphenoid (jxi.s.) has enlarged in its basi-temporal wings, in correspondence with 

 the enlarging ear-capsules. 



There is only a stapes in the middle ear ; no epi-hyal element has appeared. 



In accordance with the out-turning of the quadrate condyles, the lower jaws (fig. 7, 

 mk.) are longer; but the whole arch is largely extended by the strangely altered form 

 of the original suctorial "horseshoe" formed by the "lower labials" {l.l.). Now, they 

 are nearly straight, and stand across between the ventral ends of the mandibles. 

 These latter bars are invested on their inner and lower face by the rudimentary 

 "articulare" («'-.). The maxillaries {rnx.) also have appeared above. 



Sixth Stage. — 3. Half metamorphosed larva q/'Rana ? sp. (India) ; If inch long ; 



tail, ■§• inch; all the legs free. 



The less important specific modification here seen (Plate 4, figs. 8-10) is the flat, 

 wide cranium ; the more important morphological changes are self-evident. The 

 transversely extended auditory masses, and the wide flat occipital arch, are now largely 

 becoming ossified by the two pairs of perineural centres (pr.o., e.o.) that gradually 

 more or less occupy the occipital, periotic, and post-sphenoidal regions, protecting those 

 parts of the head where the 5th to the 10th nerves emerge. The rest of the cranial 

 " barge " is well chondrified, but has no bone as yet ; the roof bones {/.}).) are still 

 large; the floor bone (po.s.) is a short, blunt, dagger, with a very wide "guard," 

 and a very short handle. 



Over the nasal roof the nasal bones (n.) are now crescentic shells of bone ; the pre- 

 maxillaries and maxillaries (px., mx.) are now well developed, but the vomers are not 

 apparent. A thin squamosal (s(/.) like a " pre-operculum " runs do\\ai the suspensorium, 

 and the dentary (fig. 10, (/.) has appeared on the outside of the mandible, distally ; 

 the coalesced " lower labial " is becoming ossified, and the dentary is grafting itself 

 upon the endosteal patch, so as to form the " mento-Meckelian bone ;" the " articulare " 

 (ar.) is lengthening with the jaw. 



The fore parts of the trabecula), now completely confluent with the nasal roofs, are 

 broken up into a remarkable cervicorn structure. The inner angle of each is now the 

 finger shaped, curved " pro-rhinal" (p.rh.), a,nd the outer angle, which finishes the 

 nasal floor in front, is divided into three lobes, one upper and two lower, that turn 

 outwards, and are imbedded in the fore end of the maxillary. The two pairs of upper 

 labials (u.V.u.l-.) are quite nonnal now, perfecting the antero-external edge of the 

 nostril (fig. 8). Behind the jjro-rhinals a pair of "fenestroe" (fig. 9, )i.n.) are seen. 



The " pre-palatine " (j^r.jjo.) has now escajied far away from the trabecular angle in 

 frnut ; it is a sharp falcate process at the fore end of a narrow band of cartilage which 

 runs backwards until it is now opposite the prootic bone ; there it ends in the suspen- 

 sorium, which has become a stunted, triradiate cartilage, one-third of its former length. 



