174 BULLETIN OF THE 



ersten ein dritter in der Abgrenzung begriffen " 1^ Here the second pair 

 of protovertebrse, the only pair as yet distinctly formed, is quite at right 

 angles with the main axis of the chick, while the only one obliquely placed 

 — the posterior — -is not as yet fully formed, and therefore cannot be 

 the first formed. Thus we see that, where there is but one pair of proto- 

 vertebree, it makes an oblique angle with the middle line ; where there 

 are three pairs, the third as }et but half defined proto vertebra is 

 oblique ; and where there are live pairs, obliquity belongs to the last 

 two puirs;, of which tlie fifth is not as yet distinctly separated from the 

 surrounding mesoderm. It seems, therefore, reasonable to suppose that 

 the characteristic of obliquity to the middle line belongs rather to the 

 protovertebra just formed, or forming, than to the protovertebra which 

 of any given number was first formed. 



Figure 41 represents a chick with seven pairs of protovertebrse, of 

 which the fifth is marked as first. Figure 50 represents a chick with 

 nine pairs, of which the fourth is designated first; and the author 

 naively remarks, that " das Bild (Fig. 50) ist aufgenommen worden, um 

 an einem zweiten Beispiele die Constanz der Erscheinungen an den 

 Urwirbeln z\i demonstriren." Four pairs of protovertebra^ developed 

 in front of the first in the less advanced chick ; three pairs developed in 

 front of the first in the more advanced chick. Shall we not call this 

 variation, rather than constancy] I think enough has been said to 

 show that neither the size of the protovertebi-re, their relative distance 

 from the primitive streak, nor yet their obliquity to the main axis, is 

 a sufficient ground to warrant a decisive answer to the question in 

 regard to the order of their development. 



It may be that Prof Kupffcr has other reasons than those which he 

 mentions for thinking it highly probable that the fourth or fifth pair 

 of protovertebrae is the pair first developed ; but in so fiir as the argu- 

 ments ho advances are concerned, one may well hesitate to accept his 

 conclusions until they be supported by further evidence. 



I have studied one hundred and fifty or more chicks, and from their 

 external appearance I have been able to make out little in regard to the 

 order in wliich the fir.st protovertebr;c are formed. The length of the 

 chick at this early stage varies ; the shape and relative length of 

 the primitive streak vary ; the outline of the area pellucida is seldom 

 exactly the same in any two chicks. Tn fact, tliere seems to be no fixed 

 point froni which to measiire. But I have been more fortunate in the 

 study of serial sections. From these I learn that the cleft which sepa- 



1 Kupffer, p. 172. 



