Philosophical Transactions of the Royal Society of London Volume 167, PT. 2 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1878 Excerpt: ...have the surrounding parts cunningly built over and around them; sockets are sunk in the skull for their reception, albeit they are free themselves. A temporary change in the direction of the axial nervous mass in front, its mesocephalic flexure, whereby the straight embryo is formed into a crozier-like body, this, of necessity, is modifying the growth of the axial skeleton as long as the head is thus bent. Now the beautiful researches of Mr. Balfour show that the notochord becomes (in the Selachians) shaped like a sheep-hook during the period of embryonic growth; and my own researches (" On the Skull of the Shark and Skate," Trans. Zool. Soc. vol. x. plate 35) show that the part of the mesoblastic plates just in front of this bend grow more rapidly than the part behind, from which they have been, as it were, dislocated. Time has now to be considered in the morphology of the skull; and parts that start first and grow quickest generally overshadow the later and slow-growing parts. We thus get a morphological anachronism--some elements of the skeletal structure standing still and waiting, apparently suppressed, until the proper nick of time occurs in the age and growth of the animal. But for the modifications undergone by the cephalic structures, the vesicular condition of the neural axis, the development of the organs of special sense, &c, the two plates that run along the sides of the notochord might have been chondrified at one and the same time from end to end of the animal, a little slowness being allowed for the extreme ends. This would not have been materially affected by the somatomic subdivision of the tracts; they might have been separated into moieties for each vertebra, or, obliterating the earlier divisions, chondrification may be i...