CLEAVAGE 



257 



as to the organs and parts of the fully formed organism. Cells 

 may be vestigial, rudimentary, and the like, in the same way 

 that organs may be. The three or four successively formed 

 quartets of micromeres or even an individual cell, for example 

 that known as 4d, can be identified and homologized both in 

 origin and in fate, in the phyla Platyhelminthes, Annulata, 

 and Mollusca (Fig. 122). Wilson has written ("The Cell," 

 etc., page 416): "Thus we find that the cleavage of polyclades, 

 annelids and gasteropods shows a really wonderful argeement 

 in form, yet the individual cells differ markedly in prospective 

 value. In all of these forms three quartets of micromeres are 

 successively formed according to exactly the same remarkable 

 law of alternation of the spirals; and, in all, the posterior cell 

 of a fourth quartet lies at the hinder end of the embryo in 

 precisely the same geometrical relation to the remainder of the 

 embryo; yet in the gasteropods and annelids this cell gives rise 

 to the mesoblast-bands and their products, in the polyclade to a 

 part of the archenteron, while important differences also exist 

 in the value of the other quartets." (It should be added that 

 in the Polyclad, Planocera, the particular cell mentioned gives 

 rise to mesoblast also.) 



Such conditions also illustrate how the facts of embryology 

 may have a certain value, often very great, as evidence upon 

 phylogenetic problems. 



Often these similarities of structure can be carried back into 

 the pre-cleavage stage, and in the uncleaved zygote or ovum 

 before fertilization, substances can be identified which later 

 become contained within restricted groups of similar cells. 

 So that cleavage is in part to be regarded as a process by which 

 specific substances or regions of the egg become segregated in 

 different regions of the embryo, where each continues its 

 normal differentiation during later developmental stages and 

 gives rise to specific tissues or organs. In other words the 

 process of differentiation is not limited to the later stages of 

 development following cleavage; it occurs during and even 

 preceding cleavage. In AscariSj for example, each of the two 

 cells resulting from the first cleavage is specific; in other 



