THE IXADEQUACY OF THE CEEL- THEORY. 1 09 



theon'. A certain grade of organirjatioi as the result of 

 heredity rather than of cleavage is conceded for annelid 

 development, and for all forms, in so far as future characters 

 are foreshadowed in cleavage stages. This is a limited appli- 

 cation of the view which I believe holds true of all eggs, eve/i 

 before cleavage begins. It will be easy to show that the very 

 facts generally relied upon to disprove the existence of organi- 

 zation in the egg furnish very strong evidence in support of it. 



The question as to the presence of organization is not set- 

 tled by the form of cleavage. Eggs that admit of complete 

 orientation at the first or second cleavage, or even before cleav- 

 age begins, are commonly supposed to reflect precocioaslv 

 the later organization, while eggs, in which such early orien- 

 tation is impossible, are supposed to be more or less completely 

 isotropic and destitute of organization. When the region of 

 apical growth is represented by conspicuous teloblasts, the 

 fate of which is seen to be definitely fixed from the moment of 

 their appearance, we find it impossible to doubt the evidence 

 of organization, or "precocious differentiation," as it is con- 

 ventionally called. When the same region is composed of 

 more numerous cells, among which we are unable to distin- 

 guish special proliferating cells, we lapse into the irrational 

 conviction that the absence of definitely orientable cells means 

 just so much less organization. 



Cell-orientation may enable us to infer organization, but to 

 regard it as a measure of organization is a serious error. The 

 organization of a vertebrate embryo cannot be said to be 

 less advanced than that of an annelid embryo, because it 

 lacks the unicellular teloblasts which the latter may possess. 

 The regular holoblastic cleavage of the mammalian Qg^ is 

 evidently no index to its grade of organization. The more 

 carefully we compare the cleavage in different eggs, the more 

 clear it becomes that the test of organization in the Q:gg does 

 not lie in its mode of cleavage, but in subtile formative pro- 

 cesses. We find the most unlike forms of cleavage issuins: in 

 the same remarkable form-phases ; for example, the primitive 

 streak of mammalian and avian eggs ; and conversely, we find 

 identical forms of cleavage leading to fundamentally different 



