MUSEUM OF COMPARATIVE ZOOLOGY. 175 



rates two protovertebrae is not at the time of its first appearance a 

 continuous break extending throughout the width and depth of the 

 vertebral plate. As a first trace of segmentation, there appears, in a 

 line transverse to the axis of the chick, a succession of slight depressions 

 on the dorsal surface of the vertebral plate. In a series of sagittal 

 sections these depressions appear and disappear, a single depression 

 seldom occupying more than two or three sections. As the chick grows, 

 these depressions gradually deepen and become more continuous, while 

 immediately below them a slight upward curve appears in the ventral 

 boundary of the mesoderm. A little later, a distinct line appears, con- 

 necting these two indentations, and separating the cells of the undiffer- 

 entiated mesoderm from those which are about to form a proto vertebra. 

 The latter now take on the spindle shape that characterizes the proto- 

 vertebral cell, their long axes becoming gradually radial as regards the 

 protovertebral body. Soon there appears here and there a break in the 

 line that bounds the forming protovertebra, but, as a rule, cell prolonga- 

 tions, in a horizontal plane somewhat below the middle of the protover- 

 tebra, bind the new protovertebra, on the one hand to the undifferentiated 

 mesoderm, and on the other to the protovertebra last formed. 



The formation of a distinct and separate protovertebra takes place so 

 gradually that a single long section shows the process in all stages of 

 progression. Passing from the fully formed protovertebrae through four 

 or five in process of formation, one reaches the undifferentiated meso- 

 derm, in which a faint dorsal depression gives the first indication of the 

 place where a protovertebra will ultimately be cut off. 



This being the case, it is evident that, provided the order in which 

 the protovertebrae are formed is in every chick the same, and pro- 

 vided that a protovertebra which has once started to develop contin- 

 ues to develop regularly, then, to determine the order in which they 

 have developed requires but a sufficiently complete set of sectioned 

 embryos. 



In development, as in adult life, individual chicks may be expected, 

 within a certain limited range, to vary from the typical chick. Still, 

 if we find certain conditions to prevail in a large number of chicks, 

 without exception, we are warranted in assuming these conditions to be 

 normal. The proto vertebras are not formed until the mesoderm in the 

 region of the somites has grown to be quite compact, and is as many as 

 four cells deep, so that when the cell layers first take on their concen- 

 tric arrangement they lie in two layers about the central axis of the 

 protovertebra. I have never seen a protovertebra whose walls were 



