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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 

 Fertebrata. 



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, (b) 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 



