172 BULLETIN OF THE 



As my own investigations lead me to a conclusion at variance with 

 that of Prof. Kupffer, I have examined the photographs with care, 

 and am pleaSed to find that even in point of size the protovertebrse 

 which I consider to appear first may well dispute the claims of Prof 

 Kupffer's first protovertebrse. In fact, the criterion of size among 

 objects of such varying outline admits of a wide range of interpreta- 

 tion. It is not enough to judge from the superficial area, we must also 

 take into account the depth. Moreover, it often happens that a large 

 protovertebra appears opposite a small one, in which case size evidently 

 does not determine seniority, since opposite protovertebrse are known 

 to appear at about the same time. If precedence were established by 

 size, then we should expect to find a gradual decrease in size from first 

 to last ; yet this is not what we do find, and Prof. Kupff'er does not 

 affirm that any protovertebra is second to be formed because second in 

 size. Perhaps I am laying too much stress upon the matter of size, 

 since what Prof. Kupffer says in this regard is quite conditional. He 

 affirms only that, if size determines age, then we must consider this or 

 that protovertebi'a first formed. 



Moreover, it is not clear upon what Prof. Kupffer founds his opinion, 

 that the second cleft in the mesoderm appears anterior to the first,^ 

 unless it be founded upon the relative distances of these two clefts from 

 the anterior end of the primitive streak, as compared with the space 

 which intervenes between the end of the primitive streak and the first 

 mesodermic cleft in an embryo where only one cleft has yet appeared. 

 This, at least, is the argument used in regard to Figure 25, loc. cit., which 

 represents a chick with four distinct pairs of protovertebrse, and a fifth 

 faintly marked off" in the mesoderm posterior to the others. Here the 

 author considers the third from the anterior to be the one first de- 

 veloped, but allows that one might hesitate to decide between the third 

 and fourth : " denn die Entfernung des hintersten Spalts von dem vorde- 

 ren Ende des Primitivstreifs, harmonirt mit der Entfernung des ersten 

 Spalts vou demselben Punkte in den Figg. 20 und 22." (In Fig. 20 

 only one cleft has appeared in the mesoderm ; in P'ig. 22, there are but 

 two.) "Mag man nun aber den einen oder anderen der beiden hinteren 

 Urwirbel als den znerst abgegliederten auffassen, jedenfalls geht aus 

 den Figg. 24 und 25 hervor, dass die Segmentirung zunachst rascher 

 nach vorn als nach hinten vorschreitet, und ist darnach die Angabe von 

 Foster und Balfour zu berichtigen, die behaupteten dass die nachst- 

 folgenden Urwirbel hinter dem ersten Paar entstiinden." ^ 



1 Kupffer, p. 172. 2 Hjjd.^ pp. 173^ 174. 



