AND THE LOWER ANIMALS. 27 



the intermediate leaflet or lamella. These groups 

 of organs do not commence their development at 

 the same period, nor do they at first present their 

 characteristic features. . Before assuming the latter, 

 all undergo transformations. Now, unless we were 

 to begin a special treatise on embryogeny, it would 

 be impossible to give more than a general and 

 abridged account of these phenomena here; and, 

 moreover, we must be cautious in selecting our 

 examples. We shall, therefore, limit ourselves to 

 such facts as will enable us to form a general con- 

 clusion applicable to the entire animal kingdom. 



Those who have observed the successive appearance 

 of the various organs during the development of the 

 embryo, state that the first structures apparent in 

 mammalia are those most characteristic not only of 

 the animal kingdom generally, but of the sub -kingdom 

 Vertebrata ; viz., the vertebral column, cranium, and 

 the great nervous masses which they contain. The 

 question arises, are these the earliest structures in all 

 animals ? In answering this query, if we were to 

 depend on the results of direct observation alone, we 

 should at once reply in the negative. It is unquestion- 

 able that other portions of the animal apparatus, such 

 as the skin and locomotive organs, are the first seen ; 

 but then the nervous system, which is usually con- 

 sidered.- to affect the vitality of the others, is only 

 observed when in a very advanced condition. Possibly 

 it is the first to be developed, but owing to its extreme 

 transparency, we are unable to detect it with the aid 

 of our present optical apparatus. 



In mammalia, the heart accompanied by the blood- 

 vessels makes its appearance at an early date, and 



