ON THE THEORY OF THE VERTEBRATE SKULL 579 



with that of the skull, unless we analyse very carefully, and distinguish 

 from one another, the successive steps of that process.'^ 



I. 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 "lamins 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 

 haemal 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 description, 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 

 <;ertain 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 

 separated 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 

 signification. 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 

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



1 See Note VL for the details of the development of the spinal column in Vertebrata 

 generally. 



p r 2 



