DEVELOPMENT OF THE CHICK. 211 



arteries of the bag. Besides, it is much more vascular than any of the 

 other membranes, which is jnst the reverse of what we should imagine, 

 if it did not answer that purpose \ 



Of the Formation of the Parts of the Chick. 



As the parts which act in both stages differ very considerably in 

 their structure, the structure of the first not being adapted to the 

 economy of the second, we have an opportunity of investigating those 

 changes which may be said to give us the gradual formation of parts 

 till completed. The heart is the only visible acting part, and the con- 

 struction of that viscus in the very young is not similar to that of the 

 full-formed. From hence we can have its formation through its various 

 changes. 



The first parts that are visibly formed may be said to be the brain 

 and spinal marrow, although we may conceive the heart and vascular 

 system is also formed, suited to such a state, and that it is co-existing, 

 but not seen, because transparent, while the brain, &c. is opake, and can 

 be rendered much more so ; by which means it becomes still more evident; 

 for if the brain, <fcc. was transparent, the heart woidd be the first visible 

 object from its motion, and afterwards [from its] becoming reddish. 



1 [The vascularity of the external fold of the allantois, the porosity of the shell, 

 and the difference in the colour of the blood passing from the chick to the allantois, 

 to that returning from the allantois to the chick, give the highest probability to the 

 opinion expressed in the text of the respiratory function of the allantois, and of the 

 necessity of access of air to that membrane through the pores of the shell, for the 

 development of the chick. 



The experiments byErman, commenced in 1810, and published in the 'Isis' of 

 Oken for 1818, were performed with an apparatus by which the requisite heat could 

 be applied to a fertile egg in a presumed vacuum, or in an atmosphere of artificial 

 gas ; but the apparatus being defective in regard to the luting used to cement the 

 bell-glass to the brass-plate employed, Erman's arguments, that oxygen was not 

 necessary to incubation, are inconclusive. 



His experiments, repeated by Viborg, with the substitution of a more effectual 

 luting, were followed by the opposite result. Oxygen was found to be essential to 

 development, and atmospheric pressure, afforded by the medium of other gases, as 

 hydrogen and carbonic acid, was followed by no appreciable change in the cicatricula 

 subject to the incubating temperature. 



The requisite pains and precautions, which the present advanced condition of 

 chemical science enables the experimenter to put in practice, appear to have been 

 effectually taken by Dr. Schwann, who has made the question, " De Necessitate Aeris 

 Atmospherici ad evolutionem Pulli in Ovo incubito," the subject of a most able and 

 valuable inaugural thesis, published at Berlin in 1834. From which it appears that 

 the development of the embryo in the common fowl may go on without oxygen in 

 the ordinary course to the fifteenth hour, and that the life of the germ is not de- 

 stroyed till between the twenty-fourth and thirtieth hour, but that the presence of 

 oxygen is essential to further development.] 



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