Mr. T. H. Huxley on the Theory of the Vertebrate Skull. 433 



1. The primary changes of form exhibited by the blastoderm in 

 the region of the spinal column, are, in all the Vertebrata whose 

 development has yet been studied, precisely the same. Two ridges, 

 the " laminae dorsales," bounding a narrow elongated groove, rise 

 up and eventually unite with one another so as to enclose a cavity — 

 the neural canal. External to the junction of the laminae dorsales 

 with the blastoderm, the latter is converted more or less completely 

 into the "laminae ventrales," which become incurved, unite, and 

 eventually enclose the visceral cavity. 



A transverse section of the embryo in this state shows a very thin 

 and narrow median plate, separating the neural canal above from the 

 lisemal or visceral canal below, and passing on each side into thick- 

 ened masses of blastoderm, which give rise to the laminae dorsales 

 on the one hand, and to the laminae ventrales on the other. 



For convenience of ^lescription, I shall term the median plate the 

 "diaphysial plate," and the lateral ridges the "paraphysial thick- 

 enings." 



2. The primary histological differentiations which take place in 

 the rudimentary spinal column just described, are the same in all 

 Vertebrata, 



A long filament, composed of indifferent tissue, makes its appear- 

 ance in the middle of the diaphysial plate, and constitutes the 

 notochord, or chorda dorsalis. 



Next, the substance of the paraphysial thickenings undergoes a 

 certain change of tissue at regular intervals, so that they acquire a 

 segmented appearance ; solid, broad, darker masses of blastema lying 

 opposite one another in each paraphysial thickening, and being sepa- 

 rated by clear, narrow interspaces. 



These segments are what the Germans term " Urwirbel," or 

 "primitive vertebrae ;" a somewhat misleading name, as they are in 

 every way distinct from what are commonly understood under the 

 name of " vertebrae," even if we use that word in its broadest sig- 

 nification. Professor Goodsir's terms of Somatomes for the segments 

 and Metasomatomes for their interspaces, appear to me to be well 

 worthy of adoption as the equivalents of these " Urwirbel." 



3. The next step in the development of a vertebral column is the 

 histological differentiation of the somatomes. Leaving out of con- 

 sideration the epithelial and other minor tissues, it may be said 

 that each somatome gives rise to (a) epiaxial muscles, (6) a nerve 

 and its ganglion, (c; the blastema for a vertebral centrum and its 

 neural and haemal arches, and (d) possibly hypaxial muscles ; while 

 the metasomatome becomes for the greater part of its extent an 

 "intermuscular septum." 



It is unnecessary for my present purpose to trace out particularly 

 the development of any of these parts, except the centrum and its 

 arches. 



The blastema, which is specially intended for these parts, appears, 

 in a distinct form, first, in the paraphysial thickenings, and then 

 extends inwards above and below, so as gradually to enclose the 

 notochord in a sheath, while, externally, it passes in the posterior half 



Ann. ^ Mag, N. Hist. Scr. 3. Vol. iii. 28 



