DEVELOPMENT OF THE SKULL OE THE COMMON FROG. 
139 
“ parostoses ” for some days, being formed in a thick web of fibrous blastema ; they soon, 
however, apply themselves to the overlying cartilage of the basis cranii, and become 
ectosteal in relation to it. 
Again, there can be no better instance of an ectosteal sheath than the shaft-bone of a 
young Sea-turtle’s rib (“Shoulder-girdle and Sternum,” pi. 12. fig. 5); but in a short 
time the bony matter spreads into the surrounding fibrous tissue and into the overlying 
derm, affecting that tissue outside much faster than the cartilage within (ibid. fig. 6). 
Kathke had shown this long before, in his work ‘ Ueber die Entwickelung der Schild- 
kroten,’ 1848. 
In this case that which is primarily a true ectostosis becomes parosteal during deve- 
lopment, and then spreads into the substance of the skin, forming a dermosteal layer. 
That which is remarkable in the “Anura” is the paucity of bony plates as compared 
with Osseous Fishes and Eeptiles, and also the long time that the ossification of the 
fibrous bony layer keeps independent of the calcification which takes place somewhat 
later in the superficial cells of the hyaline cartilage within ; this latter is “ superficial 
endostosis,” as in the Sharks and Eays*. 
Here, in the Amphibia, both tailed and tailless, there is no original distinction to be 
seen between a parostosis and an ectostosis, and therefore the question as to whether 
the fibrous bones are ectoskeletal or endoskeletal has to be determined arbitrarily, by 
comparison with their counterparts in Osseous Fishes and in the abranchiate Vertebrata. 
That this distinction is merely arbitrary, as far as fibrous bones are concerned, is 
evident from what I shall have to describe in this paper ; for in the Frog the dentary , 
which is nearly always a reliable parostosis in the other great groups, forms here an 
ectosteal sheath to Meckel’s cartilage ; whilst the articulare, which everywhere else 
bears an ectosteal relation to the upper part of that rod, is in the Frog a mere splint 
applied to its surface. In the early condition of the larvae of the “ Amphibia Urodela,” 
the bony plates which appear in the palatal region are all similar films of bony deposit 
in delicate tracts of fibrous blastema ; and so far there is no distinction as to exoskeleton 
and endoskeleton between the parasphenoid and vomers on the one hand, and the palato- 
pterygoid plates on the other. 
In my mode of illustration, however, I shall continue to colour those bony plates 
yelloiv which, as a rule, are the immediate setters-up of ossification in the cartilaginous 
endoskeleton ; and the plates which normally keep free from the cartilage will be left 
uncoloured, as though they were truly exoskeletal ; they are, indeed, the connecting 
bond between the two systems. 
Structure of the Frog's Skull, First Stage. — Embryos from 2 to 3 lines long ; extending 
in time from 2 days before to 2 days after hatching. 
It will be necessary to describe both external and internal characters in their early stage; 
* See Rolleti’s paper (p. 114) in Stricker’s ‘ Human and Comparative Histology/ translated for the New 
Sydenham Society, by H. Power, 1870. 
