no BIOLOGICAL LECTURES. 



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

 with that of the mollusc or the annelid, where "■ cells Jiaviiig 

 precisely the same origin in the cleavage, occupying the same 

 position in the evibryo, and placed under the saute mechanical 

 conditions, may nevertJieless 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 Qg^ 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-division, may be demonstrated in almost any 

 Q^%. Watch the expansion of the blastoderm in tlie pelagic 

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

 the axial concentration of nuitcj-ial, 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 rr //-differenti- 

 ation, no more disproves the existence of definite organization 

 in the case of the egg than in tlie case of h\-dra. 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 tlie first two or four blastomeres, wlien 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 blastomcre continues for a while to form cells 

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

 fests the w^hole unit-power of the organism, 1 see no reason to 

 conclude that the case is fundamentally different. In either 

 case the part has the ])ower of reorganizing itself into the 

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



