CLEAVAGE IN ANNELIDS. IO9 



To avoid misunderstanding, let me say here that in accepting 

 this principle of Lillie's, and its application by Conklin, I can 

 hardly follow the latter author in assuming as complete a cellu- 

 lar differentiation as he does. Inasmuch as we know that pro- 

 toplasmic continuity undoubtedly exists between blastomeres, 

 we are hardly at liberty to suppose that differentiation is com- 

 plete at the time of cell formation. In using the term " differ- 

 entiation " I mean that a certain amount of material destined 

 to form a certain organ is accumulated at a definite place, even 

 though this material at the time of its separation may be in an 

 undifferentiated condition. For example, I do not believe that 

 at the time of formation /[.d is necessarily mesoderm, but that it 

 is destined to develop into mesoderm ; and, according as it con- 

 tains a larger or a small amount of material, the mesoderm of 

 the body will be large or small. 



This evidently is but a limited application of the principle 

 stated by Lillie, and it would be possible in the ontogeny of 

 Podarke to find many illustrations of the law which might, 

 perhaps, strengthen the position I have taken here. Inasmuch 

 as differentiation in cleavage forms the subject of another 

 lecture in this course, I shall not attempt to describe them, 

 but will refer to Dr. Lillie's lecture for additional evidence in 

 favor of the theory. 



Quite recently, as said before, it has been shown that the 

 cell 4d is not, as was originally supposed, purely mesodermal, 

 but that in some animals it constitutes in addition a certain 

 amount of entoderm. It might be supposed that the large size 

 of 4d in these forms is due to this double, function. Apart 

 from the fact that in certain annelids, Amphitrite} Polymnia, 

 Arenicola, no such entodermal portion has been found, — and 

 yet here the cleavage is very unequal, — is the further consid- 

 eration that in no case is the amount of entodermal tissue there 

 present sufficient to account for the difference in size. In 

 Aricia there is but one small entodermal cell budded off from 

 4d on either side ; while in Nereis, though some three or four 



1 I can hardly agree with Wilson that the small cell described by Mead as 

 budded off from 41! during its migration inward is entoderm, inasmuch as its 

 permanent position is at the anterior end of the germ band. 



