N'o. 4. — Studies on the Primitive Axial Segmentation of the Chick} 



By Julia B. Platt. 



A. — The Successiox of the Protovertebr.e. 



While studying the general development of the chick in connection 

 with my special work upon the segmentation of the medulla and the 

 origin of the cranial nerves, I came upon the following sentence in 

 Balfour's Comparative Embryology : " The first somite arises close to 

 the foremost extremity of the primitive streak, but the next is stated 

 to arise in front of this, so that the first formed somite corresponds 

 to the second permanent vertebra." ^ A foot-note calls attention to 

 the fact, that " further investigations in confirmation of this widely 

 accepted statement are very desirable." 



Prof. His is hardly more definite. He says : " Die Urwirbel, welche 

 zuerst entstehen, sind, wie dies v. Baer bereits richtig erkannt hat, nicht 

 die vordersteu Halswirbel, sondern es bilden sich vor den zuerst ent- 

 standenen Wirbeln im 5 und 6 Stadium noch einige fernere. . . . Ich 

 mochte mit v. Baer vermuthen, dass ihrer jederseits zvvei entstehen, 

 mdglicher Weise ist indess diese Schiitzung zu niedrig gegriflfen."^ 



I believe that the " Photogramme zur Ontogenie der Vogel, von C. 

 Kupffer und B. Benecke," is the only other publication in which there 

 is either confirmation or refutation of the opinion advanced by Balfour, 

 in regard to the order in which the protovertebra3 are formed. From 

 the explanatory remarks with which Kupffer and Benecke accompany 

 their beautiful photographs of chick embryos, one is led to think that 

 the first protovertebra to appear is at least the fifth in the series, count- 

 ing from the anterior. In determining the order in which the proto 

 vertebra arise, they have been principally guided by the difference in 

 size found to obtain among the protovertebrse of the same individual. 

 The largest is considered the first. 



1 These studies were pursued in the " Annex " Laboratory at the Museum of 

 Comparative Zoology, Cambridge, Mass., under the direction of Dr. Howard 

 Ayers. Zool. Laboratory, No. 14. 



2 Vol. II. p. 16L 



* Untersuchungen iiber die Erste Anlage des Wirbelthierleibes, pp. 82, 83. 

 VOL. XVII. — NO. 4. 



