XX 
ON THE NATURE OF THE EARLIEST STAGES OF 
THE DEVELOPMENT OF ANIMALS 
Proceedings of the Royal Institution of Great Britain, vol. ti. 1858-62, 
Dp 315-317. (friday, February 8, 1861.) 
THE lecturer commenced by giving a general description of the 
structure and singular properties of the animal organism, termed 
Pyrosoma Gigantcum, a specimen of which, taken by Capt. Callow in 
the North Atlantic, had been forwarded to him by Admiral Fitzroy, 
in the autumn of 1859. 
Not only had his investigations enabled the speaker to verify the 
most important of the statements made in his memoir on Pyrosoma, 
published in the PAzlosophical Transactions for 1851; but they had 
revealed peculiarities in the mode of reproduction of the animal, of 
great interest from their bearing on some of the most difficult 
questions of embryology. 
In order to render the importance of these new facts obvious, 
it was necessary to premise a concise statement of our present, know- 
ledge with regard to the early stages of animal development. To 
this end the structure of the fowl’s egg was described, and the effects 
of incubation were traced, so far as was necessary to prove that the 
chick takes its origin from the cicatricula, or blastoderm. 
It was next pointed out, that we owe the discovery of this 
important fact to the great Harvey, who, in his “Exercitationes de 
Generatione Animalium,” demonstrated with perfect clearness, firstly, 
that the chick is developed from the cicatricula, and not, as had been 
supposed, from the chalaza, or other parts; and secondly, that the 
process of development is an “ epigenesis,” or gradual addition of new 
parts to those already formed. 
