EMBRYOLOGICAL HISTORY. 155 



for no other branch of science is able to lead to any 

 conclusions as to the union of the great branches of 

 the animal kingdom into a common trunk. 



All animals start together. At the beginning of 

 life the highest and lowest alike consists of a simple 

 single-celled ovum. This is the universally recog- 

 nized starting-point. Interpreting this, therefore, 

 the embryologist says that it means that we are to 

 assume as the starting-point of life of the animal 

 kingdom a single-celled ancestor, or protozoan, who 

 lived long before the earliest record which the rocks 

 give us. Next, we find that this ovum divides into 

 a number of smaller parts, each consisting of a single 

 cell, and all at first seemingly alike, cases where they 

 are unlike being due to secondary modifying circum- 

 stances. This is called segmentation, and is a stage 

 in development universally found in all animals. It 

 is not alike in all, for the disturbing presence of food 

 in the egg often prevents a regular division. Some- 

 times the whole ovum divides, sometimes part of it ; 

 sometimes the segments are of equal size, sometimes 

 of different sizes. But after a careful examination 

 of all cases and circumstances connected with them, 

 the conclusion seems to be that they are all modifi- 

 cations of the simplest type, and the regular equal 

 division may be considered as primitive. Now, this 

 division can mean but one thing to the embryolo- 

 gist. It is the repetition of a similar process taking 

 place long ago in the primitive unicellular ancestor 

 of animals, which is supposed to have divided in 

 like manner, and thus to have formed a colony of 

 unicellular animals. This same process is known to 

 take place to-day in some low forms of life. 



