342 Mr. W. K. Parker on the Development of the [Feb. 13 y 



ethnoidal masses ; for, further forwards, at the mid-line, there is a 

 large cavity filled with fat.* In front the trabecule end in two short 

 cormia, but there is no "pro-nasal " rod. 



In the snake (" Snake's Skull," Plates 27 — 33) the cranial part of the 

 notochord shortens rapidly, and is invested with a very thin secondary 

 sheath. The trabecule are manifestly merely fore-growths of the para- 

 chordal tracts : there is no inter-trabecular tract, and even tho 

 " septum-nasi " is formed by the two trabecular crests uniting with 

 the inner edge of each nasal roof. The snake has a post-pituitary 

 wall, which is well developed in all the " Sauropsida." 



In the high- skulled lizards the inter-trabecular cartilage appears, 

 and largely contributes to the formation of the partition wall in the- 

 f ore part of the head ; it does not, however, appear, wedging in between 

 the trabecule below, as in the Chelonians, nor has it any pre-nasal 

 growth in any kinds I have worked out. 



But the Chelonians shed most light upon these parts, and my 

 recent work at this type has made many things clear to me that for a 

 long time have remained unexplained. 



The segmentation of the pro-chordal from the para-chordal region 

 is both secondary and temporary : it did not exist in my first stage, and 

 had vanished in the fourth, namely, in embryos three-parts ripe ; these,, 

 which were the size of a horse-bean (third stage), corresponded with 

 what is seen in the adult salmon. 



The embryos of Strutliio camelus come very close to Ghelone as to 

 the inwedged position of the inter-trabecular tract, which appears 

 below, and makes the base of the orbital septum carinate. 



This isjnot seen in the other " Ratitai " nor in the " Carinatse ;" but I 

 find it in the ethnoidal region in the chick (" Fowl's Skull," Plate 81). 



Also in having large orbito-sphenoids the African Ostrich comes 

 nearest to Ghelone. Neither of these types, nor the other " Ratita3," 

 show the segmentation of the perpendicular ethnoid from the septum 

 nasi, which is constant in the " CarinateB," and is seen to begin in 

 Lizards. 



In Mammals (Pig's Skull, Plate 28, fig. 8), the para-chordals 

 chondrify before the pro-chordals ; the latter, however, are never sepa- 

 rated from the former. The " inter-trabecular " wall dominates in 

 the front of the pituitary space, but the thin, flattened, vertical tra- 

 beculee diverge at a certain point to form the huge orbitosphenoids, 

 exactly as in Chelone (Plate 33, fig. 6). 



The shortening of the skull by the fold of the mid-brain, and the 

 high post-clinoid wall growing up into the hollow, are just alike in the 

 turtle and the pig. 



* Dr. Milnes Marshall has shown me that the want of symmetry seen in my 

 salmon embryos was artificially produced by the spirit ; they should have been re- 

 moved from the egg before they were preserved. 



