178 BULLETIN OF THE 



Still the new protovertebra, is little more than a half-protovertebra, for 

 it is much smaller than those immediately behind, and opens anteriorly 

 to the mesoderm of the head, into which it passes with no distinct boun- 

 dary. These peculiarities of protovertebra b are evident even in the 

 entire chick, if examined at this stage. 



For some time I thought it possible that this incomplete protover- 

 tebra might be formed from protovertebra a, but this cannot be the 

 case, for I have traced its growth from a few cells grouped together 

 anterior to a, up to the time when the ninth nerve, passing behind 

 the ear capsule, crosses so near to this protovertebra as to leave no 

 intervening tissue from which another protovertebra could be formed. 

 That another protovertebra is not formed anterior to b is also evinced 

 by the fact, that in no subsequent stage does the line of protovertebra? 

 end anteriorly in a complete protovertebra, or in a protovertebra less 

 complete than b. If b became complete, and the tissue anterior to b 

 developed into an incomplete protovertebra, it would be possible to find 

 the stage in which this change took place. 



In Kblliker's " Entwicklungsgeschichte des Menschen und der ho- 

 heren Thiere," there is a figure * representing an " Urwirbel ahnlicher 

 Korper vor der Gehbrgrube, der von einem Ganglion und zwei Nerven 

 gebildet wird." Another figure 2 represents an embryo with two large 

 cell masses at the beginning of the protovertebral line, which Kolliker 

 calls " Urwirbeln ahnliche Massen." He supposes them to represent 

 the " vereinigte Anlage der Ganglion des Glossopharyngeus und Vagus." 

 The shape of these last mentioned protovertebra-like structures, and 

 their relative distance from the ear capsule, lead me to think it highly 

 probable that their posterior wall is formed by the incomplete protover- 

 tebra (b) of which I speak above, while their anterior wall is formed by 

 the ninth nerve. However this may be, since Kolliker affirms these 

 structures to be at least in part ectodermic, they offer no objection to the 

 limitation which I place upon the mesodermic protovertebrae, of which 

 alone I speak. 



My conclusions are, therefore, that the first break in the mesoderm 

 occurs anterior to the first protovertebra, and that two protovertebra? 

 (or, more correctly, one and a half) are slowly formed anterior to the 

 first mesodermic cleft, in the time occupied by the formation of six or 

 seven protovertebrae posterior to that cleft. With regard to the num- 

 ber of protovertebrae occurring anterior to the first, my work confirms 

 the estimate of His and Von Baer. 



1 Loc. cit., p. 430. 2 Loc. cit., pp. 142, 143. 



