MUSEUM OF COMPARATIVE ZOOLOGY. 173 



Now, Figures 20, 22, 25, are said to be enlargements of the ante- 

 rior portion of Figures 19, 21, 24, respectively. In the last mentioned 

 figures, the distance from the posterior extremity of the primitive streak 

 to the head fold varies but little, while the primitive streak alone, in 

 that chick which has three or four more protovertebrse than the other 

 two chicks, is shorter by the width of those protovertebrse. Comparing 

 the pictures, and allowing for a difference in total length, we find that 

 the width of three protovertebrse has been added in Figure 24 from the 

 region of the primitive streak to that of the embryo, and this added 

 space we find occupied by protovertebrse. This being the case, does it 

 not seem quite reasonable to suppose that the posterior vertebrae are 

 developed later than the anterior, in the space once occupied by the 

 receding primitive streak, — especially since we know the important part 

 played by the primitive streak in the formation of the mesoderm 1 I do 

 not attach much value to this argument, nor do I know if it would hold 

 true for other chicks which might be photographed ; I would merely 

 say that these three photographs, which are compared by Prof. Kupffer 

 for the purpose of determining the first protovertebra, are capable of 

 leading to conclusions quite different from those at which the author 

 arrives. 



Prof. Kupffer further remarks : " An den Figg. 33 und 34 sind einige 

 Einzelheiten hervorzuheben. Man geht wohl nicht fehl, wenn Man 

 das in der Contourzeichnung mit u' markirte Urwirbel-paar [of five 

 pairs, the fourth] als das erst entstandene auffasst. Abgesehen von der 

 Grosse [it does not seem to me larger than either the second or fifth 

 pair] spricht der Umstand dafiir, dass die Axe der Urwirbel dieses Paares 

 nicht senkrecht auf der Medianlinie steht, sondern eine leichte Neigung 

 noch hinten zeigt. Es harmonirt das mit der Richtung der beiden 

 ersten Spalten, Figg. 19-22." 1 Let me call attention to the fact, that 

 the fifth pair of protovertebrse in Figures 33 and 34 shows also a 

 " Neigung nach hinten," but is evidently not therefore considered " das 

 erst entstandene." 



I allow, indeed, that the obliquity of the angle made by the fourth 

 and fifth pairs of protovertebrse with the main axis of the chick in 

 Figures 33 and 34 corresponds well with the obliquity of the angle in 

 Figures 19 and 22, where, as will be remembered, but one pair of pro- 

 tovertebrse is developed, and there is therefore no doubt about its being 

 the first ; but how in regard to Figure 23, where " der erste Urwirbel 

 ist . . . deutlich begrenzt, vor demselben ein zweiter und hinter dem 

 1 Kupffer, pp. 175, 176. 



