300 



ME. W. K. PAEKEE ON THE STEUCTTJEE AND 



simple platform it will be easy to follow the metamorphosis of the primordial parts, 

 even in the Mammal, where such changes are most of all displayed, and to compare 

 and harmonize them with the lesser degrees of transformation to be seen in Fowl, Frog, 

 and Fish. 



Second Stage. — Embryo Fig, 1 mch long. 



Most of my illustrations of the complete embryonic skull will be made from a some- 

 what more advanced stage than this ; but this second stage is of great importance in 

 illustrating the morphology of the facial arches and auditory sacs*. 



In this embryo chondrijication has fairly set in, although the cells of the hyaline 

 cartilage are still close together, quite as close as in the nidus of the vomer in the 

 next stage, or the tissue in which the rostrum of the parasphenoid is developed in the 

 embryo bird (" Fowl's Skull," Second Stage, Plate lxxxi. fig. 7, r.st.). Ossification has 

 commenced also, and can just be seen in the nidus of the vomer, maxillary, and dentary ; 

 this last is the forwardest of the bony plates (Plate XXX. fig. 2, d.). Beginning at the 

 snout we see that the alse nasi have chondrified, and that the retral trabecular horns 

 (Plate XXX. fig. \,al.n., c.tr.) have coalesced with them: the little papular prenasal 

 cartilage [p.n.) is well seen in this front view ; beneath this, a little further back, the 

 stroma becomes dense on each side and forms the premaxillary territories, and is ready 

 to ossify. In the deepest and widest part of the ethmoidal region, a vertical section of 

 which (Plate XXX. fig. 2) shows the commenced ingrowing of the proper turbinal folds, 

 we see now that the descending nasal roofs and the ascending trabecular crests have 

 all coalesced together to form the large mesoethmoid [m.eth.). A long scoop-shaped 

 territory lies immediately under the trabecular base of this septum, and this granular 

 tract is undergoing endostosis; it is forming the vomer [v.). Far on each side, above a 

 rudimentary tooth-pulp, is a faint trace of the maxillary [mx.). In this "schizognathous" 

 stage the root of the tongue is seen at no great distance from the freely exposed vome- 

 rine region, and the oral cavity {m.) has here steep sides, in the walls of which the 

 primary palatal bars {])-])g-) are seen as compressed granular plates. On each side, 

 below an inferior tooth-rudiment (t.), a large mass of nascent cartilage is seen, having a 

 kidney-shaped section ; and inside this a round rod of cartilage is seen, converging towards 

 its fellow of the other side as it passes forwards. If my observations had ended here, 

 the thick slab of granular tissue, with its incurved edges, would have merely been noticed 

 as the proper dentary territory or nidus of the mammalian mandible ; it is more than 

 this, as the next two stages will show : the " rod " is Meckel's cartilage {mk.), the shaft of 

 the first postoral arch. The dentary bone itself appears in this section, and is of a 

 rich rose-colour in the preparation, one stained with carmine ; the tissue around the 

 osseous deposit is becoming colourless, like Meckel's rod, for the carmine scarcely tints 

 the cartilage. The other postoral bars are shown in this section ; the " cornu minor " 



• From a largo number of exquisite sections of this stage I have only made the six illustrations here given ; 

 for what the rest show is better seen in a somewhat more advanced stage, the morphological level being essen- 

 tially the same. 



