THE UNIT OF STRUCTURE 33 



Only indirectly is their well-being affected by that of the body 

 as a whole ; only accidentally is the death of the body the 

 occasion of their death. The same might be said of such para- 

 sites as the " blood-worms " of Egypt, or the trypanosomes (the 

 cause of " sleeping sickness ") of Equatorial Africa. Occasion- 

 ally, in the rare disease lymphocythaemia leucocytes multiply 

 exceedingly, not, apparently, in response to a call for their 

 presence in large numbers, but in defiance of the needs of the 

 economy, and with baneful results. To the indispensable 

 services which wandering cells render, frequent reference will be 

 made. In the present connection, and while we are searching 

 for the principles of construction of the animal body, it would 

 be desirable, if we could do so, to define the status of wandering 

 cells. If they entered the body from without, they would be 

 parasites of commensal type, intruders who share in the food 

 and shelter of the body in return for service. But they do not 

 enter from without. They are cells of the growing body which, 

 detaching themselves from the cells which are forming tissues, 

 assume a wandering life. They are not to be recognized in the 

 embryo until development is considerably advanced. Their 

 origin is far from clear, but histologists believe that, although 

 they are not recognizable as wandering cells in the earliest 

 stages of growth, they, or rather their parent cells, are set 

 apart at a very early date. Probably they are not formed 

 in the embryo proper, but in the " extra-embryonic area," 

 from which they emigrate into the embryo. In this sense 

 they come in from outside. But, after all, the extra- 

 embryonic area equally with the embryo is a product of 

 the ovum. Looking at the body as a whole, we recognize 

 a common life, a soul in Aristotle's sense, which inhabits the 

 framework of fixed tissues ; and at the same time we see 

 a multitude of independent cells, each an organism in itself, 

 produced, like amoebae, from similar independent cells by cell 

 division, absorbing the body fluids, consuming invading germs 

 and fragments of decaying tissues, dying, disintegrating, in 

 their turn absorbed. Wandering cells are autonomous in the 

 largest sense. 



All multicellular plants and animals are formed by division 

 of a primitively single cell, the segments remaining in contact. 

 As the scale of life is ascended, the cells which are massed 



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