130 SHOULDER-GIRDLE AND BREAST-BONE. 



XVIII. If we now review what has been advanced concerning the structure and development 

 of the Sternum in various Saurians, we may conclude from the results of direct observation that in 

 their position, composition, and form they bear a close resemblance to the similar structures in allied 

 Vertebrate animals, and that they also originate and develop in a similar manner. In regard to the 

 development of the Sternum in the Saurians the following general facts may be admitted : 



1. In a few of these animals, especially in those of the Genera Acontias, Chirotes, and Chamaleo, 

 the Sternum is developed from two morphological elements corresponding in form and size, com- 

 mencing at a distance from one another in the lateral halves of the body, and subsequently coalescing. 

 A short time after their appearance they form well-marked cartilage. In the majority of Saurians, 

 however, a third piece joins itself to the two similar symmetrical lateral pieces which are here also 

 formed. This third piece [inter-clavicle] takes origin in the middle line of the body, beneath the 

 symmetrical pieces, and subsequent to their union. It never presents the character of cartilage at any 

 period, but undergoes ossification. 



2. The two symmetrical morphological elements of the Sternum originate for the most part close 

 under the several ribs, unite these together at their lower ends, and continue persistently in intimate 

 connection with them. In the At}'pic Scaly Reptiles, however, they originate at a distance from the 

 ribs, and never enter into intimate connection with them. 



3. As a rule these two elements coalesce with one another after they have come into lateral oppo- 

 sition. But in Aconlias meleagris, in which, speaking generally, the Sternum remains in the lowest 

 grade of development, they indeed approximate closely, but never actually coalesce. 



4. When these parts are only of small extent, proportionately to the whole body in the first 

 instance, on subsequently coming into contact with one another, they completely coalesce and form, 

 after undergoing considerable increase in accordance with the general growth of the body, a simple and 

 continuous plate, as occurs in the general Ophisaurus, Anguis, and Pseudopus ; but if they are of pro- 

 portionately moderate length they rarely unite with one another throughout their whole length, as for 

 example in Gongylus ocellatus, but usually remain divided to a greater or less extent, either posteriorly 

 where they are narrow, or also frequently further forwards, where they increase in breadth ; in conse- 

 quence of which, in a state of complete development, there may be remarked in the Sternum a pair of 

 horns posteriorily, and more anteriorly a fontanelle. 



XIX. The posterior half of the anterior segment of the Sternum [inter -clavicle] projects in 

 many Saurians in which it has attained only a small breadth in proportion to its length, as, for 

 example, in the Varanida?, beneath the other or larger piece of the Sternum, and thus forms an inferior 

 keel beneath the other. It appeared desirable, therefore, to me to determine whether, in Birds and in 

 those Saurians which possess a keel to their Sternum, this arises in a similar manner to the anterior 

 Sternal piece of Saurians, in the perichondrium of the sternal plate, and subsequently, as in a few 

 Saurians, coalesces with this plate. 



But both in the embryos of Fowls, and in young Sparrows, I found that the keel of their 

 Sternum was formed of true cartilage, and that this was continuous without interruption with 

 the still cartilaginous plate of the Sternum, of which it appeared to be an outgrowth. The same was 

 the case in older embryos of Bats, and in newly born Moles, whose Sternum already possessed a small 

 keel. 



XX. A few anatomists have regarded the anterior or accessory of the two pieces out of which, 

 in the majority of Saurians, the Sternum is composed, as equivalent to the 'manubrium sterni' of Mam- 

 mals. But the 'manubrium sterni' in Mammals is developed (if the two originally divided lateral halves 

 of their Sternum which correspond to the symmetrical morphological elements of the Sternum of 

 Saurians have united) out of the most anterior parts of the same, whilst these parts increase in breadth 

 to a greater degree than the longer portions of the already mentioned lateral halves of the Sternum 



