324 



REPTILIA. 



period the vitelline sac, together with its re- 

 maining contents, is gradually taken into the 

 cavity of the abdomen through the umbilical 

 aperture, and before the egg is hatched has 

 entirely disappeared. 



In Fig. 246. the condition of these parts is 



Fig. 245. 



Vitelline apparatus of the Fetus of a Viper at a 

 more advanced period, showing the Yolk partially 

 entered into the Abdominal Cavity, 



a, the yolk-bag; b, the amnion ; a', portion of 

 the yolk-bag which has passed through the um- 

 bilical canal into the cavity of the abdomen ; d, d, 

 ductus vitello-intestinalis running forward to open 

 into the intestine ; g, continuation of intestinal tube ; 

 k, the kidney. 



Fig. 246. 



The Embryo of a Viper just before it is hatched, 

 showing the Condition of the Vitelline System at this 



The letters a. c, g, h, I, indicate the same parts as 



in fig. 244.; b, remains of the vitelline sac, taken" 

 out of the abdominal cavity, in which it had now 

 become completely closed; d, the ductus vitello- 

 intestinalis shrunk into a very short canal ; /, /, the 

 stomach; h, i, rudiments of the ovaria; k, the 

 kidney. 



represented as they appear just before the egg 

 is hatched, the vitelline sac being already 

 completely introduced into the abdominal 

 cavity ; the yolk of the egg, b, now reduced 

 to a very small size, is now seen to commu- 

 nicate with the intestine^ g, by the extremely 

 short passage, d, to which the long ductus 

 vitello-intestinalis, represented in Fig. 244., is 

 now reduced. 



Tegument ary System. In all reptiles the 

 blood is cold, and the general temperature of 

 the body corresponds with the imperfectly 

 oxygenated state of the circulating fluid ; 

 instead, therefore, of being clothed in hair or 

 feathers, their bodies are invested with plates 

 or scales of horny cuticle, better adapted to 

 their manner and mode of life. In lizards 

 the cuticular covering is cast off at intervals 

 in small detached portions ; but in serpents, 

 where it forms a thin continuous stratum that 

 envelopes the whole surface of the body, it is 

 cast off in a single piece. 



Beneath the cuticle, the skin of reptiles 

 presents the usual structure, consisting of the 

 corium, and of an interposed mucous layer, 

 upon which the various colours of the surface 

 of the body depend, which in some species 

 are of great "brilliancy. 



The chameleon and other lizards are remark- 

 able for the changes of colour of which the 

 surface of the body is susceptible, in accord- 

 ance with the intensity of the light to which 

 it is exposed, or the nature of the locality 

 in which it happens to be placed. These 

 changes seem to a considerable extent to be 

 voluntary, and under the control of the animal ; 

 and various hypotheses have been framed in 

 order to account for them, but without any 

 very satisfactory result. 



In the rattlesnakes (Crotalus) the cuticle 

 in the vicinity of the tail presents a very 

 peculiar modification of structure : instead 

 of forming imbricated scales, as in other parts 

 of the body, it is arranged in a series of 

 rings loosely connected together, so as to 

 constitute the remarkable rattle which cha- 

 racterises these dangerous snakes. This 

 singular organ is made up of many pieces, 

 from one to thirty or more, which are per- 

 fectly similar to each other in their form, and 

 are articulated together by a very beautiful 

 mechanism. 



The piece of the rattle immediately con- 

 nected with the body seems to be moulded 

 on the last vertebra of the tail, which it en- 

 closes, and from which it is only separated by 

 an interposed layer of the dermis or true 

 skin, by which it is secreted. Its surface 

 presents three circular elevations correspond- 

 ing to three protuberances : of these the first, 

 or that nearest to the body of the animal, is 

 the largest ; the other two rings are encased 

 in the succeeding piece, which is connected 



