in the Pectus of Vertebrated Animals. 269 



bryo appear to arrange themselves, there is nothing in the api-, 

 pearance or structure of the germinating spot, so far at least as 

 has been ascertained by the accurate investigations of the most 

 eminent physiologists, which assimilates any of its parts to those 

 of the foetus or perfect animal, the formation of which it precedes. 

 We seem entitled, therefore, in the present state of our know- 

 ledge, to regard these germs as wholly invisible — perhaps entire- 

 Ijf" imaginary, since their existence is only inferred from pheno- 

 mena which occur during the development of the embryo. 



II. We have had an opportunity also of observing how very 

 different the parts of the embryo are on their first appearance 

 in the ovum, from those parts which they represent, and into 

 which they are transformed at a later stage of foetal life, or after 

 birth ; and we have seen how gradual the change is by which 

 liiik transition is effected. 



III. From the important place which the cerebral and circu- 

 lating organs occupy in the perfectly formed animal, many have 

 IveTieved that the formation of the brain and heart precedes that 

 of all other parts ; but it has been shown that, though these are 

 among the organs which appear to be most highly developed 

 or perfected soonest, yet before the commencement of their for- 

 mation, and certainly before any partSj which may with justice 

 be compared, in function or structure, to these organs, are pro- 

 duced, the substance is deposited from which the head, trunk, 

 and extremities are formed. , i 



The heart has also been supposed to take its origin in conse- 

 quence of some influence derived from the brain and spinal 

 cord ; ^nd many have imagined that the development of the 

 greater number of organs in the body, follows necessarily the 

 presence of the heart, or of certain bloodvessels ; but the phe- 

 nomena of the development of these parts appear to prove such 

 ideas to be erroneous, and to show that we are as yet ignorant 

 of any particular influence which the pre-existence of one part in 

 the early stages of its advancement exerts on the formation of 

 another. Many observations, indeed, show that the brain and 

 heart are nearly simultaneously produced, and that in all those 

 parts which may be examined vvith case, a certain quantity of 

 their parenchyma is formed, before they receive the bloodvessels 

 which, at a later period, serve for their nutrition.- 



