l 



CLEAVAGE 231 



consequently, being free from the influence of the dead and 

 inert deutoplasm, will be able to divide sooner. 



While these laws are often applicable to the processes of cleav- 

 age in a general way, the exact study of cleavage in a great 

 variety of forms has disclosed very numerous exceptions and 

 contradictions. On the whole we may say that such laws, though 

 still retaining a limited applicability, are chiefly interesting as 

 indicating the attempt to refer the phenomena of cleavage to 

 the grosser mechanical relations of cell structures. It is now 

 clear, as we shall see later, that other factors are of greater im- 

 portance in determining the form and rhythms of cleavage. 

 The fundamental "organization" of the ovum, which is not only 

 morphological but physiological as well, is the primary factor in 

 determining the characteristics of cleavage. The numerous 

 "exceptions" to these laws of cleavage are definitely related to 

 both the organization of the ovum and also to the structural 

 and functional characters of the later stages of development, 

 since these too are primarily determined by the same organiza- 

 tion factor. 



Most of the conditions which form exceptions to these rules, 

 and are therefore deviations from the simple and regular 

 form of cleavage like that of Synapta, may, following Wilson 

 ("The Cell," etc.), be grouped under three heads. (1) Unequal 

 Division. While this is usually related to differences in deuto- 

 plasmic content, there are many instances where no such rela- 

 tion can be made out and the inequalities must be explained 

 upon other grounds (e.g., the micromeres of the Echinoids, the 

 teloblasts of certain Annelids and Molluscs). (2) Cell Dis- 

 placement. This may result from the atypical position of the 

 spindle or from the shifting of blastomeres after they have been 

 formed. Often the individual blastomeres are only lightly 

 held together since they normally show a tendency, often very 

 marked, to assume a spherical form. Under these conditions 

 they might tend to assume a position described by the law 

 (Plateau's) of "least surfaces" or "minimal contact," according 

 to which a group of elastic spheres, like bubbles, held together 

 and yet free to move, tend to become arranged in such a way 



