DEVELOPMENT OF THE SKULL IN THE liATRACHIA. 29 



Third Stage.— I (continued). Tadpole of Rana clamata (B) 3 inches 5 lines long; 

 tail 2^ inches long; hind legs, 1 \ inch long. 



In this stage (Plate 4, figs. 1-4) we see some metamorphic progress beyond the last ; 

 the general form of this as compared with Stage A (same species) is not much altered 

 (see Plate 2, figs. 5-7), but there are some new parts, and some changes in the old. 



The time, indeed, has been short during which these changes have taken place, for 

 these Tadpoles even at their height, as larva3, were beginning rapidly to be trans- 

 formed. The exoccipitals {e.o.) are lai-ger, but the prootics are not visible ; outside the 

 frontal region the premaxillaries {px.) have appeared in addition to the three investing 

 bones seen in the last stage. But the thing of most importance is the rapid growth of 

 the " intertrabecula " as a vertical nasal septum (s.«.), and the solidification of the mem- 

 branous roof of the nasal sacs into a pair of ear-shaped nasal cartilages (na.) whose 

 antero-external notch forms the inner boundary of the outer nostril {na., e.n). These 

 elegant shells of cartilage are becoming confluent with the top of the septum internally, 

 and with the ethmoidal " wing" (ct/.e.), behind : they are true " paraneural," and 

 answer to the "sclerotics" and " periotics." 



Another change is the retreating of the orbitar process away from the side of the 

 skull, so that the "post-palatine" rudiment is exposed (pr.p., pt.pa.) ; the "pre-palatine" 

 rudiment [i)r.pa.) in front of the internal nostril (i.n.) is also further from the trabecula. 



The fore margin of the chondrocranium is very similar to what it will be 

 eventually, but the rounded end of the intertrabecula is still distinct from the 

 nasal roofs [s.n., na.), and the cornua trabeculse (c.tr.) are still undivided at their end. 



The great upper labial {u.l.) has lost the fenestra that showed signs of division, 

 right and left, and is now, more than ever, one large saddle-like plate.'"' 



The mandibular and hyoid arches are much in the same state as in the younger 

 larvae (Stage A), but the branchials are altering preparatory to extensive absorption. 



Compared with those of the Bull-frog (Second Stage, Plate 2, tig. 8), it will be seen 



* In this and tlie last two stages (A and B) we have a very instrnctive analysis, so to speak, of tlio 

 complex nasal capsule in the higher Vertebrates, e.g., of that in the Mammals. It is composed primarily 

 of the trabecules with their lateral (antorbital) wings, where the cranium closes in iu front of the vertical 

 " intertrabecular " middle wall ; of the nasal roofs which may grow down into side walls ; and of the rich 

 outgrowths of turbiiials that grow from the front of the antorbital wings of the ethmoid {upper ami 

 middle lurhinid.<) ; and from the inside of the outer wall (iii/cn'or tuihinaJs). 



tDther turbinals may spring from the roof (nasal turbinals of the Rodents), or inside the outer nostril 

 {nli-nasal turbinals of Pig and Bird). 



Then the narial valves are formed of segments or subdivisions of the upper labials, the whole structure 

 being roofed by the nasals, floored by the vomer or vomers and palatine plate of the mamillaries, and 

 walled in by the maxillaries and premaxillaries. 



Lastly this complex labyrinth of cartilage may, itself, become bony, to a greater or less degree, and even 

 may coalesce, more or less, with the investing bones. 



Anyhow, the plan and patteni, and the numhered elements of this labyrinth, are to be been iu their fii-st 

 simplicity in the nasal region of the Tadpole. 



