CHAPTER X. 



CORRELATION OF GROWTH. 



In considering that elementary form of reproduc- 

 tion, in which the small mass of protoplasm which 

 constituted the parent organism simply divides and 

 gives rise to two similar but smaller organisms, we 

 saw that the plan of development, or rather the 

 co-ordinations which determine future development, 

 are not destroyed. The same is true of the germ- 

 cells, or embryos, of many higher animals. They 

 may be divided into several parts, and each produce 

 a complete animal. The embryo of the small marine 

 animal, Pyrosoma, divides regularly into four parts, 

 each of which produces an individual. And in the 

 group of the Tunicata, to which Pyrosoma belongs, 

 there are many forms in which, as soon as the em- 

 bryo is differentiated into the three cell layers, — 

 hypoblast, mesoblast, and epiblast, — it divides itself 

 into several similar embryos, which may develop into 

 complete individuals, or may, perhaps, first divide 

 again before attaining maturity. We see this same 



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