132 HAY. [VOL. I. 



Kolliker's " perichordal centra," are more properly chordal than 

 are the " chordal centra " themselves; nevertheless, they adopt 

 Kolliker's term in the sense employed by him. If in this con- 

 nection it is necessary to keep up the distinction between the 

 notochord and its sheath, it would be better to adopt the pro- 

 posed term, " auto-centra." 



One of the most interesting propositions advanced by Gadow 

 and Abbott is that relating to the origin of the skeletogenous 

 tissue. According to them, cells are given off from the ventral 

 portion of the protovertebra, which cells form a pyramidal mass 

 whose base lies against the lower border of the notochord, 

 but whose apex rises to its upper border or higher. It has 

 hitherto been held that all, or nearly all, of the skeletogenous 

 layer has been thus derived. But these authors have discov- 

 ered another source of skeletogenous cells, and, to judge from 

 the diagrams presented on page 188, at least one-half of these 

 cells originate there. This new fountain of formative materi- 

 als is found in the upper portion of the protovertebrae. Cells 

 are there emitted which form an inverted pyramid, the apex of 

 which extends to the lower border of the notochord. The 

 dorsal pyramid grows downward in front of the ventral pyramid 

 belonging to the same protovertebra. The bases and the 

 apices of these two sets of pyramids are capable of developing 

 into cartilage, an intermediate zone not attaining this capabil- 

 ity. The result is that the bases give rise to the neural and 

 haemal arches (basidorsals and basiventrals), while the apices 

 give origin to the " intercalated cartilages " (interdorsals and 

 interventrals). From this it will be seen that the two dorsal 

 ridges of the embryo, from which arise the neural arches, are 

 in small part only derived from the lower portion of the proto- 

 vertebrae while the two ridges below the notochord receive 

 some materials from the upper portion of the protovertebrae. 



It may appear remarkable that so great a mass of cells is 

 contributed to the skeletogenous layer by the upper ends of the 

 protovertebral plates without its having been discovered long 

 ago. The production of the dorsal contingent proceeds not 

 earlier than the other, and many observers have witnessed the 

 upward growth of the ventral cells. 



