128 SHOULDER-GIRDLE AND BREAST-BONE. 



nine pairs. The majority of these are attached to the body of this piece, but a few to the anterior 

 part of its horns to wit, one pair in Crocodilus vulgaris, two pairs in Gavialis Schlegelii, C. aculus, 

 C. biporcatus, Alligator lucius, A. sclerops, A. cynocephalus, A. punctulatus, and A. palpebrosus. 



16. The result of researches which I have instituted in embryonic specimens of Lacerta agilis 

 and Crocodiles of different ages * has shown that the Sternum in these Reptiles consists also, as follows 

 from my former researches on Birds and Mammals, 3 in the first instance of two proportionately very 

 small, band-like, straight bodies, alike in form and size, and composed of a firm cellular mass. These 

 in the anterior half of the body diverge from one another, are split upon the two lateral halves of the 

 body, uniting on each side with several consecutive ribs ; on the whole they lie widely separated from 

 one another, and are connected by a portion of the inferior uniting skin. Gradually they approximate, 

 and after the lapse of some time come quite anteriorly, and subsequently also more posteriorly, into 

 contact with one another. Their approximation depends upon the circumstance that the intervening 

 portion of uniting membrane becomes constantly smaller by absorption. On the other hand, the 

 peculiarly and primarily extremely small lateral walls of the trunk, the anterior halves of which 

 likewise present a raphe beneath the band-like lateral halves of the Sternum, constantly become 

 broader. From this circumstance, moreover, the two large pectoral muscles attached to them, which 

 likewise primarily lie far removed from one another, increase in breadth, both absolutely and relatively ; 

 but the ribs, especially all the true ribs, increase in length, not absolutely alone, but also in proportion 

 to the length of the body. As the lateral halves of the Sternum are first in contact anteriorly, so 

 they coalesce gradually from before backwards. In reference to this process there is so far a 

 remarkable difference between these Reptiles and the higher Vertebrata, that in the latter the lateral 

 halves of the Sternum coalesce throughout their whole length, whilst in the former this only takes 

 place to a partial extent. In these Reptiles they remain divided posteriorly to a moderate extent, 

 and these separated lateral portions form in these posterior parts, in adult life, the two horns of the 

 Sternum. Moreover, in Lizards, in the anterior part, though certainly only to a small extent, no 

 coalescence occurs of the two lateral halves of the Sternum, and at a later period this spot appears, after 

 a portion of the Sternum has attained a considerable breadth, as a space or fontanelle of considerable 

 size filled up with fibrous tissue. Such a fontanelle was apparent already in nearly mature embryos 

 of Lizards ; but was in them of proportionately much smaller size than in adults. 



The horns of the Sternum never project in Lizards over the last pair of (i. e. the last true) ribs 

 attached to them ; in Crocodiles, however, they reach beyond the ribs at first, though only to a slight 

 extent, increase in the extent to which they project over them, constantly further and further, and at 

 the same time curve outwards. The lateral halves of the Sternum have already at the time when 

 they have coalesced by their anterior extremities a somewhat greater breadth anteriorly than 

 elsewhere. In the Lizards the wider portion is of greater extent than in the Crocodiles. In the 

 further progress of embryonic development these anterior broader parts of the Sternum increase in 

 breadth, not only absolutely, but also in proportion to the extent of the posterior narrower portion, 

 and form the lozenged-shaped plate of the Sternum. When the embryo leaves the egg the general 

 form of this piece of the skeleton composed of the above-mentioned two pieces are already the same as 

 in adults of this species. 



The moderately dense mass, composed of closely compressed cells, of which the two lateral halves 

 of the Sternum originally consist, soon become converted after these two parts have united anteriorly 

 into true cartilage. In the Crocodiles this change takes place without interruption through 



1 I have dissected nine embryos and several young ones amongst the Crocodiles, of which the smallest embryo was two inches 

 two lines long. 



3 'Archiv. f. Anat. u. Phys. v. Joh. Muller,' 1838, pp. 3G3 366. 



