176 BOARD OF AGRICULTURE. 



is a mark that fecundation has taken place. After a short 

 time the germinal vesicle disappears, and is succeeded by two 

 cells. Each of these twin cells gives origin to two others, 

 making four. Each of these four in turn gives origin to two, 

 by which the number is increased to eight ; and this mode of 

 augmentation continues until the germ consists of a mulberry- 

 like object, the cells of which are so numerous as not to admit 

 of being counted. Together with the doubling of the cells 

 in number has been a diminution in size, and each cell is 

 found filled with the foundations of new cells into which its 

 nucleus has been resolved. 



From this germ the embryo begins to be formed. There 

 is a separation into a defined central and peripheral portion, 

 both of which at first appearing granular, subsequently are 

 found to consist of vesicles. This mass becomes in turn more 

 and more differentiated, and we have an appearance which has 

 been described as the primitive streak, and a little further in 

 time the dorsal laminae. Thus the development proceeds, 

 constantly showing greater changes and more differentiation 

 of structure. 



At first the development indicated is that of the lower 

 animals, as of the fish ; then that of the reptile, the bird, the 

 mammal ; then to the species, — variety, — and afterwards to the 

 individual. The progress is from the simple to the complex. 

 It will not be understood that the human embryo is at one 

 time a fish, at another time a reptile, etc. He merely assumes 

 the same type of structure that the bird or reptile assumes in 

 their developmental state.* 



It is accordingly seen that a full development of a part 

 into a completed structure is a complex process, in which the 

 forces are changed and are changing constantly, and only after 

 a steady continuance of developmental changes can finally 

 possess the force of the strength and character fitted for the 

 succeeding structure. The force sufficient to develop the fish 

 or reptile is insufficient to develop the man ; and when the 

 force is at that point of accumulation, through inheritance and 

 otherwise, that its strength and direction is in any way equiva- 



* For an illustration of this community of structure, as shown by development, see 

 Darwin's " Descent of Man," Vol. I., p. 15, where drawings of a human embryo and 

 of an embryo dog, at about the same stage of development, are contrasted. 



