CHAP. VIII.] LIKENESSES IN ANIMALS AND PLANTS. 271 



it approximative^, as we approach it, intellectually, on as 

 many different sides as we can, and as nearly as we can. 



To return to the question of the vertebral or non-vertebral 

 nature of the skull : the result of all the contro- Are there 



, , . cranial verte- 



versy on the subject up to the present time is that bras? 

 such vertebral nature may be affirmed in one sense and 

 denied in another, according to the line of thought which is 

 followed. 



The whole body .of every animal with a distinct skull and 

 backbone exists at first as a rounded, almost structureless 

 mass of tissue, in which the first clear indication of such 

 animal is a longitudinal furrow marking the place of the 

 future spinal marrow and brain. Beneath this furrow, a rod 

 made up of cells (the chorda dorsalis) comes to lay the 

 foundation of the future spinal column. From each side of 

 the groove a fold extends upwards, the two folds being called 

 the laminsG dorsales, and these folds, meeting together above, 

 form a canal. It is within that part of the laminee dorsales 

 which form the spine, that first the cartilages und then the 

 bones are developed which form the sides of the vertebral 

 arches. Similarly, it is within that part of the laminte dor- 

 sales which form the skull that first the cartilages and then 

 the bones are developed which form the sides of the skull 

 arches, and thus there is an undeniable similarity between 

 these two parts. Moreover, in subsequent development, the 

 bones of the skull especially in the higher animals pre 

 sent a singular reminiscence of vertebrae in the three serially 

 successive arches which they form. Certainly, if the essence 

 of vertebrae consists in their being a series of bony rings 

 fitted together, and enclosing the nervous centres along the 

 dorsal region of the frame, then it must be asserted that the 

 skull is in part composed of three bony vertebrae. 



In certain fishes the transition from the spinal column to 

 the skull is so gradual that it is easy to mistake part of that 

 column for part of the skull. Thus, in the sturgeon, the 

 cartilaginous representatives of true vertebrae coalesce into 

 one mass with the cartilaginous skull ; and in the Siluroid 



