ORGANIC DIFFERENTIATION 81 



After their first division, the blastomeres are often very 

 unequal ; the smaller, clearer, and more transparent being 

 formed of almost undiluted living substance, the granular and 

 larger opaque one contains practically all the alimentary 

 reserve. This large blastomere continues to divide unequally, 

 and forms new small, translucent blastomeres, but, at the same 

 time, the original itself divides in a leisurely fashion into other 

 blastomeres which remain granular and much larger than the 

 clear ones. The embryo, consequently, is made up of two kinds 

 of blastomeres, which are necessarily conjoint and mutually 

 dependent on the larger for the supply of the nutritive substances 

 which are needed for the increase and division of the smaller. 

 From that time the embryo forms a unit, still susceptible of sub- 

 division into cellular agglomerations possessing both elements 

 and capable of developing into distinct individuals. The egg 

 of certain Bryozoa, 1 after segmentation, can still subdivide 

 into any number of groups of blastomeres up to a hundred, 

 each of which is capable of producing a larva. The embryos 

 of some species of Lumbricus even divide regularly into halves 

 before producing the succession of segments that constitute 

 the worm. In exceptional cases this division may still be 

 partially realized in the embryos of higher animals, and it is to 

 this that many kinds of double monstrosities owe their origin. 

 But, as a rule, after a very short time it becomes impossible 

 to separate the different parts of the embryo from one another, 

 and each blastomere, from the time it is formed, appears to 

 have a particular destination. Some seem to be charged with 

 the duty of forming the anterior portion of the body, others 

 the middle portion, and still others the posterior ; some belong 

 to the left side, others to the right ; some are utilized for the 

 ventral side, others for the dorsal ; each blastomere playing a 

 distinct part in the building up of the tissues and organs of 

 the different parts of the body. If one of these particular 

 elements is suppressed, its place is not then taken by the others ; 

 the tissues and the organs that should develop from it do not 

 appear ; they are suppressed with it ; l if displaced, it does its 

 own work in the new place imposed on it. The embryo thus 

 resembles a building made of stones, which have been prepared 

 in advance to occupy one fixed place and no other. Potentially 

 it contains all the different parts of the future adult individual, 



1 Experiments of Wilson. 



