REPTILES. 129 



the whole length of the two lateral halves of the Sternum ; but in the Lizards a fibrous tissue is 

 formed out of these cellular masses at those points of the Sternum to which the horns and the shield- 

 shaped plate subsequently become movably united. An articulation of this kind I could already dis- 

 cover in embryos whose cutaneous investment could have only ! j t begun to be coloured. Calcareous 

 deposit takes place in Lizards, as well as in other Scaly Reptiles, both in the shield- formed plate and 

 also in the horns of the Sternum, at a later period, subsequent, indeed, to the time at which the 

 animal has left the shell, for in all young specimens of Lacerta agilis and of Tejus Teguixin I found 

 the parts in question in a completely cartilaginous condition. 1 



In Lizards and Crocodiles the smaller or so-called anterior sternal bone [inter-clavicle] first 

 makes its appearance after the two lateral halves of the Sternum have united to a considerable extent, 

 and have obtained a moderately thick fibrous investment as perichondrium. It originates on the 

 under surface of the Sternum in the substance of the above-mentioned fibrous tissue without any 

 primary formation of cartilage. Immediately after its first appearance, whilst yet forming an extremely 

 delicate pencil, pointed at both ends, it is composed of true bone, in which bone-corpuscles, with a 

 few canaliculi, may be clearly recognised. Nevertheless, this mass I have particularly noticed in young 

 embryos of Lizards is for some time so soft that it is easily broken down by slight pressure. At a 

 later period, when it has become more firm and hard, there may always be found, as I have seen in 

 young examples and in embryos of Crocodiles, and also in young specimens of Tejus Teguixin, a thin 

 investing layer of a soft blastema, essentially composed of elementary cells, which likewise, without 

 previous conversion into cartilage, ossifies, and serves to add to the dimensions of the anterior sternal 

 piece. Consequently this piece of the Sternum is to be enumerated amongst the so-called secondary 

 bones, or is to be considered as an investing (opercular) bone for the other piece of the Sternum, from 

 which it constantly and remarkably differs in its structure. The anterior piece of the Sternum, soon 

 after its formation, develops so as to cover the anterior part of the posterior segment [manubrium] , in 

 consequence of which the projecting part becomes surrounded and invested by a portion of the perichon- 

 drium of the other piece. Moreover in Lizards the projecting portion throws out at an early period a 

 pair of lateral prolongations, which quickly increase in length. In older and nearly mature embryos 

 of Lizards it has the same form of a cross as it has in adults, and is also similar, in the proportion of 

 its size, to that of the posterior piece, as in adults. 



XVII. In two embryos of Anguis fragilis, which were two inches three or four lines long, the 

 Sternum appeared to be much smaller, iri proportion to its length, than in adult specimens of this 

 species, and had nearly the form of a square with rounded angles ; it differed, however, from a perfect 

 square in that it was a little narrower posteriorly than in front, and that the posterior border had a 

 shallow indentation in the middle. It was composed of a thin cartilaginous plate and a still thinner 

 and much smaller plate of bone [inter-clavicle] . The latter lay under the anterior half of the former, 

 was united with it by fibrous tissue, and approximated to a triangle in form. The three edges were 

 slightly concave, two of the angles were rounded, and the remaining posterior angle was directed 

 backwards, and prolonged into a moderately long point. Bone corpuscles with caniculi could be 

 clearly distinguished in it. With the hook clavicles [coracoids] the Sternum was intimately united, 

 but it was not very closely connected with the neighbouring ribs, lying at a much greater distance 

 from them than in adult Blindworms. There cannot, consequently, be any doubt that in the Blind- 

 worms the two lateral halves of the Sternum do not, as in Lizards, Crocodiles, Birds, and Mammals, 

 originate immediately under the ribs, and unite with them, but develop at a distance from the ribs. 



These young specimens of Tejus Teguixin were three in number. A cicatrix at the umbilicus 

 was still perceptible, although they had already attained a length of nine inches four to seven lines. 

 17 



