in the Animal Kingdom. 127 



varieties of structure in the animal kingdom, are but modifica- 

 tions ofi essentially, one and the same fundamental form ?* 



Now, seeing that not only the Vertebrata, but all Classes of 

 animals, in their development, must pass thus gradually from 

 a merely animal form, to the most special forms they respectively 

 attain ; further, that the manner of development may be consider- 

 ed as essentially the same in all ; is it surprising that there are 

 resemblances between some of the embryonal phases of very 

 different animals ; and that some of the stages in embryonal life 

 of the more elaborate structures, resemble perfect states of those 

 that are less wrought out ? Could it, indeed, have been other- 

 wise ? 



Let us inquire a little more particularly into the development, 

 firstly, of the Vertebrata ; and, secondly, of some Invertebrated 

 animals. 



Firstly Of the Vertebrata. 



The layers into which the germinal membrane separates, be- 

 come, as already said, tubes. (See Plate I. fig. 6.) These 

 tubes are more or less bent towards the yolk, at each extremity ; 

 but extend the whole length of the animal, including its head 

 and tail. Therefore, out of the upper tube, constituted by a 

 union of the laminae dorsales (fig. 6, &.), are formed, the arches 

 of the caudal, lumbar, dorsal, and cervical vertebrae, the arched 

 cranial bones, and the soft parts covering all of these ; together 

 with the central portion of the nervous system. While out of the 

 under tube, constituted by a union of the laminae ventrales, (c) 

 are formed, the ribs, the soft parts of the thorax and abdomen, 

 the hyoid bone, and all that portion of the neck, anterior (or in- 

 ferior) to the vertebrae, the lower jaw, and some other parts, both 

 osseous and fleshy, of the face. The bodies of the vertebrae, 

 and the base of the cranium, are formed out of a portion of the 

 animal layer of the germinal membrane, common to the upper 

 and the under tube (fig. 6, a.) 



The central portion of the nervous system in different animals, 

 may, in its ultimate elaboration, produce very different struc- 



* Whether this fundamental form is vesicular, as it has been supposed, and 

 in favour of which opinion, several facts might be brought forward, -we can- 

 not now inquire. 



