1880. | A Sketch of Comparative Embryology. 245 
which cause it to divide into numerous cells, and cause these cells 
to arrange themselves upon the model of the parents which formed 
the egg and spermatozoa, and to imitate the peculiarities of the 
cells in each locality, making an eye where the parent had an eye, 
a gland where the parent had a gland—only the imitation is 
imperfect, the offspring is not absolutely the same as the parent. 
Evidently the fusion of the genoblast is the source of an increased 
vitality and of a formative power which is specific in each case, 
í. e., the action and result of which is predetermined. 
This marvelous formative power has always excited the inter- 
est and astonishment of naturalists. Itis one of the fundamental 
distinctions of life, since no similar power occurs in inorganic 
nature. Itis important to note, therefore, that it must enter into 
all cells, otherwise some of them would not form in the right 
place and manner. That other cells than the fertilized ovum 
contain such a power is shown by the formation of buds and 
strobila, and more strikingly by the development of pseudova. 
In the latter instance, the development begins with a cell arising 
in the ovary, and which resembles an ordinary egg very closely. 
Such cells are formed in various animals, notably in the plant 
lice, but, although they are so like eggs, the pseudova differ by 
being capable of developing intó a complete animal without 
impregnation. ` 
For want of space, it is impossible to describe the formation of 
buds and strobila, let it therefore suffice to say, that the reproduc- 
tion depends in both cases upon the separation of a cluster of cells 
(instead of a single cell or pseudovum) from the body of the pa- 
rent. This cluster grows up into a complete animal, in which the 
Structure of the parent, or sometimes of the grandparent, is imi- 
tated by the action of the formative force of the cluster of cells. 
Hence it is evident that a similar power is bestowed upon several 
cells, which is the thesis we started to prove. 
II.—SEGMENTATION AND THE FORMATION OF THE 
After the impregnation has been completed, and the two pro- 
nuclei have fused, to form the segmentation-nucleus, there usually 
follows a period of quiescence, during which no visible changes 
Occur. It is rot known whether such a period is always intercalated - 
in the course of development; but it has been observed frequently. _ 
After this pause the process of segmentation begins, which has for ee 
