IIO BIOLOGICAL LECTURES. 



results ; for example, in the egg of the polyclad as compared 

 with that of the mollusc or the annelid, where " cells having 

 precisely the same origin in the cleavage, occupying the same 

 position in tJie embryo, and placed under the same mechanical 

 conditions, may nevertheless differ fundamentally in morpho- 

 logical significance. ' ' (Wilson .) 



The most remarkable feature of avian development is the 

 primitive streak. The presence of this feature in typical form, 

 in such an egg as that of the mammal, is certainly one of the 

 most significant facts in embryology. The conclusion is here 

 forced upon us and I see no escape from it that the forma- 

 tion of the embryo is not controlled by the form of cleavage. 

 The plastic forces heed no cell-boundaries, but mould the 

 germ-mass regardless of the way it is cut up into cells. That 

 the forms assumed by the embryo in successive stages are not 

 dependent on cell-divisioil, may be demonstrated in almost any 

 egg. Watch the expansion of the blastoderm in the pelagic 

 teleost egg, the formation of the germ-ring, and especially 

 the axial concentration of material, which is so beautifully 

 illustrated in these eggs. Such developmental processes 

 are, if I mistake not, clearly indicative of some sort of organ- 

 ization. 



The formation of the whole from a part, regarded by some 

 as conclusive evidence of isotropy and correlative ^'//-differenti- 

 ation, no more disproves the existence of definite organization 

 in the case of the egg than in the case of hydra. A fragment 

 of a hydra may reproduce the whole organism ; and in so doing 

 act as a unit, not as a fraction of a unit. In the same way, 

 one of the first two or four blastomeres, when severed from 

 vital connection with its fellow or fellows, may develop as a 

 unit, not as a half-unit, precisely as Wilson insists is the case 

 in Amphioxus. 



If the isolated blastomere continues for a while to form cells 

 as if it were a half-unit or a quarter-unit, and only later mani- 

 fests the whole unit-power of the organism, I see no reason to 

 conclude that the case is fundamentally different. In either 

 case the part has the power of reorganizing itself into the 

 whole, and it makes no essential difference whether the reor- 



