M. llathke on the Development of the Chelonians. 317 



made in other Chelonians which were not entirely developed, I 

 shall endeavour to give a sketch of the manner in which the de- 

 velopment of the Chelonians takes place in general. 



After a somewhat advanced development of the extremities in 

 the embryos, the dorsal and ventral surfaces of the body are gra- 

 dually flattened, more or less, according to the different species, 

 and two lateral apophyses begin to shoot from all the twelve or 

 thirteen vertebrae of the trunk. Most of these apophyses, being 

 developed like the eight intermediate pairs of ribs, acquire in a 

 short time a very considerable length. As they are in general 

 but slightly curved, their extremities are turned more outwards 

 than downwards. Thus, by the very rapid and considerable elon- 

 gation of these ribs, the sides of the body, containing their ex- 

 tremities turned outward, are pushed much forward on each side, 

 and the trunk becomes very wide between the anterior and pos- 

 terior feet, situated at its commencement and termination. 



It is a fact as singular as characteristic of the Chelonians, that 

 their last two ribs, remarkable for their longitudinal growth, that 

 is to say, in general the eighth and ninth pairs, are turned directly 

 backward, whereas the second pair (but not in all the Chelonians) 

 have a somewhat anterior direction. The chorion then forms a 

 fold on each side, at the spot where the extremities of the very 

 prolonged ribs (second or eighth pair) are situated. This fold 

 stretching out, projects anteriorly beyond the anterior foot to 

 reach the neck, and posteriorly beyond the hindermost foot to 

 reach the tail ; lastly, it meets, on the neck and on the root of the 

 tail, the similar fold of the opposite side, and the two unite to 

 form a single circular fold, which then separates the back of the 

 sides of the body. In some Chelonians, especially the marine, 

 this fold expands slightly during the development ; in others, 

 principally in the Trionyx, it becomes extremely broad, especially 

 the part situated above the tail. Much later, that is to say after 

 the hatching of the embryos, the ribs, before remarkable for their 

 length, but up to that time, all or nearly all of a cylindrical form, 

 become also much wider. This increase in width begins from 

 the spot where the neck unites with the body, and advances 

 thence more or less toward the extremities ; it becomes so con- 

 siderable, that the bodies on all the ribs, from the complete 

 absence of intercostal muscles, are on each side in contact and 

 adhere, either perfectly, that is to say in their whole length, as 

 in the genera Emys, Terapene, Testudo, Trionyx, or almost per- 

 fectly, that is to say for the greater part of their length, as in 

 the Chelonia. The intercostal nerves and some vessels situated 

 at first between the ribs, then pass beneath them. In return, 

 the first and the last rib become much shorter than the others, 

 and always continue very narrow and thin. Their relations also 



