140 Dr Barry on the Unity of Structure 



To the law, requiring that a more fundamental type shall uni- 

 formly manifest itself before the appearance of one more subor- 

 dinate, is perhaps referrible, the formation of parts that seem 

 to answer no other purpose than the fulfilment of this law ; viz. 

 parts that either continue rudimentary through life, or not being 

 used, disappear. 



An example of the first, occurs in the appendix vermiformis 

 of the caput coecum coli, in the human subject ; of the second, in 

 the embryonal gills of land and air-vertebrata, which latter, hav- 

 ing at no period an aquatic respiration, can never use gills.* De- 

 velopment proceeds to a certain point though this point may 

 differ in different animals in obedience to the law, requiring 

 that a more fundamental type shall uniformly manifest itself, 

 before the appearance of one more subordinate ; so that the spe- 

 cial purpose to create Birds, Quadrupeds, and even Man himself, 

 is, as it were, subordinate to the more general purpose, to create 

 a Vertebrated animal. This explanation will perhaps apply to 

 all parts present in a rudimentary state alone, f 



any thing like probability in what we have proposed as an explanation of the 

 re-appearance of general characters in individual development, it has, in 

 some degree, resulted from reflection on the contents of his admirable work. 

 See the last 100 pages of his " Entwickelungsgeschichte," entitled, " Frag, 

 raente zu einer kunftigen Gesetzlehre der individuellen Entwickelung." 



* Rathke (Meckel's Archiv, 1827, p- 556.) and Von Bar, have described 

 gills, in embryos of Mammals and of Man; Huschke (Oken's Isis, 1828, Heft 

 I. p. 2.) in very small embryos of Birds. 



-f- There are, however, certain parts of structure, that arise and disappear, 

 not rudimentarily, for the fulfilment of this law ; but to serve purposes requir- 

 ed by the temporary relations of germinal and embryonal life. Such are the 

 yolk, and umbilical vesicle, the amnion, chorion, and placenta, or correspond- 

 ing parts ; the gills, fins, and tail in the tadpole, or foetus of the frog ; to 

 which examples, there might be added a host of others. 



The metamorphosis of insects, furnishes a beautiful instance of the tem- 

 porary presence of certain parts of structure, during embryonal life. Instead 

 of an appended yolk, over which the Mucous or Vegetative layer, of the Ger- 

 minal Membrane, is spread, to imbibe nourishment ; that layer, in the larval 

 state, becomes speedily a huge intestine, into which food is taken in prodigi- 

 ous quantity by the mouth. The vegetative process is, in this condition, the 

 main object. But, as the pupal state is gradually attained, growth yields to 

 transformation ; and, as Burmeister has well shewn,* the intestine is, in part, 

 metamorphosed into generative organs ; which, in the Imago, or perfect insect, 

 give origin to germs, destined to undergo like changes. 



*Lc. 



