122 T)ans(((ii<»is. — Zoology. 



A siuiukaiicuus retardatiou in the cliondrification of the 

 sternum would result m the development of a sternum formed 

 of indifferent tissue and subsequently chondrified from the 

 ribs, and from this condition of things it is but a step to the 

 earliest stage in existing Aviniota, in which the first in- 

 dication of the breastbone consists of paired patches of 

 cartilage formed by the union of the anterior thoracic 

 ribs. 



Any facts tending to show that any portion of the sternum 

 originates independently of ribs will support this view, and in 

 this connection Goette's observations on the development of 

 lizards are significant. In an early stage of Cnemidoplionis 

 each half of the sternum is a triangular patch of tissue (fig. 4, 

 st.) extendi.ig beyond the level of the third thoracic rib, but in 

 connection only with the first {Th. Bb. 1) : in other words, 

 that part of the sternum which corresponds with the second 

 and third ribs is formed independently of them, and as a back- 

 ward growth from the anterior portion. 



My own observations on Apteryx tell in the same direction. 

 In the earliest stage in which the sternum is present it ex- 

 tends backwards to the level of the third thoracic rib ; the 

 first two ribs are united to it by joints, the third is loosely 

 attached by connective tissue. In the next stage, the first 

 three ribs are attached by joints, and the fourth by fibrous 

 tissue. That is, as it appears to me, the portion of the 

 sternum corresponding to the third and fourth ribs is formed 

 by a backward growth of the anterior region and quite inde- 

 pendently of the last two ribs, the union of which with it is a 

 secondary process. 



I am disposed to consider the stages in the phylogeny of 

 the sternum to have been somewhat as follows : — 



1. Segmentation of anterior and posterior ele- 

 ments (pre- and post-omosterna) from ventral 

 ends of coracoids : there is no evidence to 

 show whether these were origiirally paired 

 or unpaired, but the former seems more 

 likely. Notidanus. 



■2. Pre- and post-omosterna arise each from paired 

 elements, which after wards unite, segmented 

 from the coracoids : the pre-omosternum 

 may be absent, and the post-omosternum 

 may be formed in part from chondrites 

 (? vestigial ribs) formed in the inscriptiones 

 stendinete {Urodcln). Amphibia. 



■ ). Disappearance of pre-omosternum : late union 

 of first pair of thoracic ribs with post-omo- 

 sternum. Hypothetical. 



