340 



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



into separate tracts the existing skates and rays show ; as the 

 tissue became more and more solid, it became, in relation to 

 the action of the mnscle-plates, differentiated into a paired chain of 

 pieces of solid cartilage for the origin and insertion of the segmental 

 muscles. 



The " basi-neural " tracts of the head in existing Yertebrata are 

 developed very differently in different types, some parts forestalling 

 others in chondrification, and yet not similarly in all. 



In the Salachians (Scyllium, Baia, Pristiurus, see " Trans. Zool. 

 Soc," vol. i, Part IV, Plates 33-42) these tracts are very broad, and 

 there are three regions made evident by the earlier or later time at 

 which they become converted into hyaline cartilage. 



The middle region chondrifies first ; this extends from the nasal 

 sacs to the middle of the auditory capsules, and answers to the 

 " trabecule " without their " cornua." 



The next tract is behind the trabecule ; it does not end close behind 

 the exit of the vagus, but runs on for a considerable distance into the 

 spine, without any sign of segmentation. 



The last to solidify its embryonic, into hyaline, cartilage, is the inter- 

 nasal; this tract is, at an early stage, narrow, for the nasal sacs are 

 immense ; afterwards these structures get much further apart, and 

 allow of the free development of the intervening cartilage. 



This overshadowed, pinched tract of the skull, breaks out into three 

 parts in front of the nasal sacs, — a pair of bilobatea, " cornua tra- 

 beculee," and the azygous " pre-nasal" rod. 



That rod is the fore part of the inter -trabecular tract ; it is but little 

 evident between the nasal roofs, and between the eyes only forms the 

 thinner middle part of the cranial floor. 



But the azygous cartilage, behind the pituitary body, forms the un- 

 divided, secondary (mesoblastic) sheath of the notochord ; in the early 

 embryos it runs on from the pituitary space to the end of the body : 

 afterwards, it is segmented at the occiput, and behind that region 

 becomes segmented (more or less) into "centra." 



Yet in the rays a large post-occipital tract is continuous, perma- 

 nently; and in both rays and sharks it shows at first signs of ex- 

 tensive segmentation in the head, for the notochord at its fore end 

 becomes "moniliform." I find fi ve joints or headings in Pristiurus, 

 and eight in Scyllium. 



In the Urodeles (" Skull of Urodeles," Plate 22, fig. 1, tr), for in- 

 stance in the axolotl-embryos at the time of hatching (my second 

 stage), show a pair of broad para-chordal cartilages that embrace the 

 fore half of the relatively huge notochord. These grow in front ol 

 that rod as small rounded horns, embracing the sides of the fore-brain 

 below. 



Afterwards these rods grow up to, and then between, the nasal cap- 



