60 Traces of Unity in the 



hence it may be expected that the peculiarities to which 

 attention has been directed are signs of immaturity, and 

 that a corresponding state of things is likely to be met 

 with at one time or other in the embryonic condition of 

 animals of higher grade. And so it is. 



The cerebro-spinal column makes its first appearance 

 as a line, or primitive streak upon the pear-shaped 

 pellucid area, or embryonic shield. Then this primitive 

 streak deepens into a groove and at the same time 

 is divided into two portions by the formation of a trans- 

 verse fold, the lower portion, which is the first to appear, 

 becoming in due time the spine, and receiving the name 

 of primitive groove, the upper portion, out of which the 

 head has to be formed, being called medullary groove. 

 Then the neural canal is formed by the gradual develop- 

 ment and coalescence of the laminae dorsales, or edges of 

 the primitive groove, and of the medullary folds, or 

 edges of the medullary groove. Then, the spinal portion 

 of this neural canal becomes divided by transverse lines 

 into segments known as protovertebrae, each of which 

 consists of two lateral masses, and a central space, which, 

 along with other spaces of the kind, enters into the forma- 

 tion of the chorda dorsalis. In the lateral masses of the 

 protovertebra, the main arteries and veins, the Wolfian 

 bodies, certain bony centres, &c., are subsequently de- 

 veloped, the history of each mass being a repetition of 

 that of the other in every particular. The place of the 

 "bodies" of the future vertebrae is occupied by the 

 chorda dorsalis, but only \hz place. Each " body" is, in 

 fact, formed by the coalescence of certain osseous 

 elements belonging to each of the two lateral portions of 

 the protovertebra, just as the heart is formed by the 

 coalescence of the vessels present in each of these two 

 lateral portions. As the bony portions of the two halves 



