MUSEUM OF COMPARATIVE ZOOLOGY. 177 



distinctly set ofiF than in the chick with a less number of protovertebrse. 

 The sections, however, do not support this supposition, since they show 

 that, while the undifferentiated mesoderm posterior to the first proto- 

 vertebra (1) is of the requisite thickness to permit of the formation of 

 protovertebrse, the mesoderm anterior to a is too thin. This I find 

 to be the case in all chicks which I have examined at this stage of 

 development. Aside from the thinness of the mesoderm anterior to 

 protovertebra a, the sections also show that even in the anterior part 

 of a the cells have not increased as rapidly as in the posterior part. 

 Consequently, this protovertebra is characterized by a peculiarly asym- 

 metrical form, which gives it, in the early stages of its growth, an 

 appearance quite different from that of the other protoyeHebrae. 

 (See Figs. 2, 3, 4, and 6.) 



Figures 5 and 5 a are taken from a series which was not cut parallel 

 to the axis of the chick, and therefore the appearance of protovertebra 

 a in Figure 5 is dekisive. These facts force me to conclude that, 

 contrary to the opinion advanced by Kupffer and Benecke, the develop- 

 ment of protovertebrse is much more rapid caudad than cephalad. In- 

 deed, after the first cleft in the mesoderm appears, dividing two forming 

 protovertebrae, the posterior mesoderm goes on continually diflferentiat- 

 ing into protovertebrse until the chick acquires its complete number 

 of protovertebrse, while it is only after three or four protovertebrae 

 have been thus formed posterior to the tii*st, that protovertebra a 

 becomes finally separated from the anterior mesoderm, after which this 

 mesoderm also slowly acquires sufficient thickness to form another pro- 

 tovertebra anterior to a. 



Figure 5 a was drawn for the purpose of showing a group of cells 

 lying between / and g. They come into section near the outer (i. e. 

 lateral) boundary of the line of protovertebrae, and extend through 

 only three sections, while it takes twenty sections to pass through a 

 protovertebra. They are, in fact, the first indication of the formation 

 of a protovertebra anterior to a. A like group of cells is not found 

 on the opposite side, nor do they occur on either side of the chick from 

 which Figure 6 is taken. But I find a similar group of cells in the 

 same position (between / and g) in the series from which Figure 7 is 

 drawn. The section represented is that which shows best the character- 

 istic radial arrangement of these cells. Here, too, but few sections pass 

 through the group. In the succeeding stages they are more marked, and 

 by the time five or six protovertebrae have been formed posterior to 1 

 these cells have become distinctly protovertebr^l in their arrangement. 



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