272 Dr Allen Thomson on the Vascular System 



of arrangement in the organs of yertebrated, and those of aver- 

 tebrated animals ; or inform us whether, among the latter, there 

 is any order or family, resembling more than the others the verte- 

 brata, which forms th^ connecting link between the simpler and 

 more compHcated class of animals. The few observations which 

 have as yet been made on this subject, by those who possessed a 

 general acquaintance with the phenomena of development, have 

 already laid the foundation of this knowledge, and have pointed 

 out some curious and interesting points of resemblance between 

 vertebral and avertebral animals, in regard to which the greatest 

 dissimilarity was previously believed to exist*. 

 ifs VI. During the development of any of the vertebra ted ani- 

 mals, as the germinating speck passes from the form of a granular 

 mass, in which it first makes its appearance, to the state of em- 

 bryo in which we perceive the rude sketches of its principal 

 organs, and gradually assumes the more perfect form of foetus 

 differing little from the adult, the animal makes a gradual transi- 

 tion from a simpler to a more complicated organization. Hence 

 has arisen the opinion, not uncommon among physiologists, that 

 the foetus, at every successive period of its development, assumes 

 the form of some animal inferior to it in the perfection of its struc- 

 ture. From the analogy which we have already stated to exist be- 

 tween the mode of development of different orders of vertebrated 

 animals, and from the gradual manner in which the complication 

 of their structure is increased, as well as from the resemblance 

 well known to exist in the general plan of their construction, it 

 will immediately be apparent that the foetus of the higher orders 

 of these animals must resemble, at different successive periods, 

 to a certain extent, the adult members of the lower orders ; but 

 as the periods at which all the organs correspond are not the 

 same, the resemblance must be considered as imperfect, and is 

 more apparent in "respect to particular organs than to general 

 structure. Many differences exist between the organization of 

 vertebrated and ayertebrated animals, of so important a nature 

 as to render any comparison such as that just noticed vague and 

 ^i^nsatisfactory at any period of the foetal development. 

 Vm VII. In regard to the formation of the heart, it seems to be 



• See Burdach's Physiol. B. ii. RaLhke's and Forchhammer's observations 

 on the deyelopnient of the craw-fish, lobster, &c. 



