Comparative Development. 29 



the human embryo, under the age of four weeks, be compared with the 

 embryos of other animals, the resemblances obtain, not merely with ref- 

 erence to the higher mammals, but with reference to all vertebrates. 

 And in its very earliest stages it cannot be distinguished even from cer- 

 tain of the invertebrate animals. On the other hand, its development 

 after the age of four weeks gradually separates it from that of all in- 

 ferior animals. Its resemblance to that of the higher ape tribes, how- 

 ever-, continues almost up to the date of birth. It is this remarkable 

 unity of early embryonic life amongst the various members of the ani- 

 mal kingdom that proves the common origin and blood relationship of all. 



But there must be degrees of consanguinity between the different ani- 

 mal tribes, and man "cannot be related to all alike. 



Assuming provisionally the correctness of the genealogical table con- 

 structed by Haeckel, we will compare the different members of the genea- 

 logical tree with each other, and note those points of resemblance be- 

 tween them in form and development, which are relied upon to prove 

 their common ancestry. ( See table, chapter 6. ) 



As previously mentioned, there are four various modes by which, 

 amongst different classes of animals, the formation of the embryo from 

 the single celled egg proceeds. The distinction between the four pro- 

 cesses relates to the manner of splitting up the cells, called segmenta- 

 tion, and to the manner of forming the cells into the plates or layers, 

 which is called gastrulation. 



There are two main divisions of eggs. In one division, called the 

 "holoblastic, " the egg is all used up in the process of segmentation or 

 cell cleavage, it being all vitalized or fertilized protoplasm. After its 

 gastrulation is completed, the further growth and development of the 

 germ plates depend upon the supply of nourishment from sources out- 

 side of the egg. 



In the other main division, the egg is called ' 'meroblastic. " In this 

 the egg is composed of two parts, the vitalized part, which is subject to 

 cleavage growth and repeated segmentation, and a yelk or deposit of 

 nutritive matter which never undergoes cleavage, but is gradually con- 

 sumed as food or nourishment by the vitalized part. 



Each of these main divisions is subdivided into two. In the first 

 subdivision of the holoblastic type the segmentation of the original 

 egg cell proceeds until the embryonic mass is reduced to a ball com- 

 posed of cells packed together like the fruity globules of the blackberry 

 or mulberry, and at this stage it is called the mulberry germ or 

 1 'morula. " In the further growth and expansion of the ball, the cells 

 form a spherical sheath or cover, and the center becomes a hollow filled 

 with protoplasmic fluid. In this stage the embryo is called a blastula. 

 One half of the cells are dark and the other half bright. 



