FIFTH LECTURE. 



EQUAL AND UNEQUAL CLEAVAGE IN 



ANNELIDS. 



A. L. TREADWELL. 



Investigations in cell lineage, the line of work contemptu- 

 ously regarded by some morphologists as a process of " cell 

 counting," have given us in the last few years some important 

 data concerning the early cleavage stages of animals belonging 

 to a number of different classes, the best-known groups being 

 the annelids, molluscs, and platodes. We are now in possession 

 of a sufficient number of facts to draw some conclusions con- 

 cerning the cleavage of the ovum and the formation of the 

 embryo. Some of the results of these investigations, bearing 

 upon questions of cell homology, differentiation, etc., have 

 been discussed by other lecturers in this course, and I shall 

 therefore limit myself to problems connected wth the early 

 cleavages. 



Concerning cleavage in general, its origin and meaning, very 

 diverse views are held by different writers. On the one hand, 

 it is maintained that there is from the beginning a definite 

 organization of the ovum, and that cleavage merely brings this 

 organization to light by isolating certain areas by means of cell 

 boundaries. On the other hand, it is argued that the ovum is 

 at the beginning isotropic, and that differentiation only begins 

 after cells are formed, and as a result of their mutual action 

 upon one another. It is maintained by some that the similari- 

 ties which appear, for example, between the cleavage stages of 

 an annelid and a platode are of as great value in establishing 

 relationships as are the resemblances which exist between the 

 so-called homologous parts of the adult animals ; in other words, 



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