Art. IV. — The Nomenclature of G/iicken Embryos for 

 Teaching Purposes. 



(With riatcs 4, 5, 6 and 7.) 



By Professor W. Baldwin Spencer, M.A. . 



[Read Thursday, May 8, 1890.] 



The method of indicating stages in chicken embryos by 

 the number of hours or days during which the egg has 

 undergone incubation is unsatisfactory, inasmuch as different 

 eggs incubated for tlie same length of time will be found 

 frequently to contain embryos which have reached by no 

 means similar stages of development. 



A more satisfactory method is to give to the successive 

 stages certain fixed and simple designations. This plan was 

 adopted by Balfour in the nomenclature of elasmobranch 

 embryos ; and, for teaching purposes, the same system may 

 be adopted with advantage in the case of chicken embryos. 



The following indicates, briefly, a series which I have 

 found convenient, and which is being adopted in connection 

 with a students' manual now in course of publication. The 

 stages are based upon the external form, and in this way 

 the embryos may, as in the case of elasmobranch embryos, 

 be classified without recourse to section-cutting, or the 

 examination of the internal anatomy. 



Each stage is indicated simply by a letter of the 

 alphabet, and to avoid confusion, the letters / and J have 

 been omitted. 



Stacjc A. — This includes the early stages, commencing 

 with the appearance of the first segmentation furrow in 

 the germinal disc, and terminating in the formation of the 

 blastoderm. This stage may be said to be passed through 

 by the egg duiing its descent down the oviduct, and before 

 incubation takes place. 



Stacje B. — The pellucid and opaque areas become defined, 

 and in the hinder portion of the former, the " embryonic 

 shield " is formed. 



