OF VERTEBRATA. 313 



p. 280, fig. 134, cr). The latter (ri) is not as yet closed over so as 

 to form a perfect tube ; in fact, even in the adult its opposite 

 edges are united by a thin membrane which is rather the sheath 

 of, than a part of, the tube itself. The eye (ey] is very large in 

 comparison to the size of the head, and, as its position indicates, 

 originates in' connection with the anterior part of the brain, 

 whereas the ear (e) is developed from a totally different region, 

 t. e.j the cerebellum. Both the eye and ear are saccular prolon- 

 gations or projections from the regions whence they originate ; 

 the one being formed into a cup-shaped body into whose mouth 

 the rays of light pour from every direction, and the other taking 

 the guise of a drum, against whose sides the reverberations of 

 the air pulsate with varying intensity. 



Beyond this stage the process of development has more 

 especial reference to the formation of a particular kind of reptile, 

 t. e., a turtle, than to the construction of a Vertebrate in general ; 

 and I shall therefore merely indicate the further growth of the 

 embryo in as brief terms as possible. The stomach and intes- 

 tine are formed by a folding together of the right and left halves 

 of the membrane in which the blood-vessels run, in such a way 

 as to leave a hollow channel extending from one end of the body 

 to the other, except at that part of it where the yolk sac is at- 

 tached, and there it lies in open communication with the latter. 

 The lungs arise as hollow, saccular prolongations of the sides 

 of the throat. The liver is a thickening of the same membrane 

 from which the stomach was formed, and is perforated in every 

 direction by a dense net-work of blood-vessels. The reproduc- 

 tive organs arise in intimate connection with the posterior end 

 of the intestine. The legs develop as direct outgrowths from the 

 right and left sides ; first as mere rounded protuberances, and 

 finally expanding into flattened pads or paws. The shield is 

 formed by a lateral expansion of the regions of the back and 

 front, with anterior and posterior apertures into which the head 

 and legs can be withdrawn. During the progress of the above- 

 mentioned developments, and the increasing size of the body 

 proper, the nutrient organ becomes gradually lessened, and 



