in the Fivtus of Vertehrated Animals. 271 



forming the rudiment of, or being converted into, the spinal canal* 

 4. The separation of the substance of the germinal membrane into 

 three layers, in which different systems of the animal appear to 

 originate. 5. The expansion of these layers of the germinal mem- 

 brane over the surface of the yolk, so as to form a new covering 

 for this part, situated within its proper membrane ; and the 

 development of blood, and of a vascular network on the surface 

 of this covering or yolk-sac. 6. The development of the head, 

 trunk, and ex-remities, from the outermost or serous layer, as it 

 has been called ; the formation of the anterior and posterior 

 extremities taking place sooner or later, according to the cir- 

 cumstances in which development is effected, and according to 

 the mode of life of the perfect animal. 7. The formation of 

 the cavities destined to contain the brain and spinal cord from 

 folds and thickenings of the same layer. 8. The development 

 of the organs of the senses from these cavities, and their 

 contained parts; the early appearance of the eye and ear 

 especially. 9. The formation of the principal circulating 

 organs, such as the heart and larger bloodvessels, from the 

 middle, or, as it has been named, vascular layer. And, 10. The 

 development of the intestinal, respiratory, and some of the prin- 

 cipal secretory organs, by means of folds and other changes of 

 the two interior, or vascular aud mucous layers of the germinal 

 membrane. These phenomena, occurring during the developmeqt 

 of the foetuses of vertebrated animals, are so nearly alike, tJiat 

 they may be considered as a strong corroboration of the opinion, 

 that the general plan of construction and arrangement of the 

 organs of all these animals is the same. 



V. The same extent of knowledge of the development of the 

 fcBtus of Avertebrated animals has not as yet been obtained ; but 

 it appears probable that, notwithstanding the great dissimilarity 

 between the adult members of this class of animals, the observa- 

 tion of die manner in which their organs originate and become 

 developed, may tend also to elucidate their connexions, and point 

 out analogies where they were not before suspected to exist. 



Such a knowledge would, I doubt not, instruct us also on the 

 imjiortant question which has lately engaged the attention of 

 two of the most distinguished comparative anatomists in France, 

 viz. whether there exists any analogy between the general plan 



