l86 CONKLIN. [Vol. XIII. 



the entire development, intrinsic factors of development are 

 more important than extrinsic ones. 



Reversal of cleavage may be due, apparently, to either of the 

 following causes: (i)it maybe produced by external mechanical 

 disturbances which compel a second division in the same direc- 

 tion, or (2) it may be caused by the precocious appearance of 

 certain organs or planes of symmetry. In most cases of normal 

 cleavage I believe it can be shown that the first cause is 

 dependent upon the second, and that the ultimate cause of 

 reversals is therefore an intrinsic one. 



All that one can afhrm concerning the so-called " law of 

 alternating cleavage " is that in early stages successive cleav- 

 ages tend to alternate in direction if uninfluenced by processes 

 of differentiation. This law of alternation is less manifest in 

 the later than in the early stages of development, and even 

 in the early stages it may be violated as soon as definite cell 

 groups, e.g., the cross, begin to appear. 



Apart from the general phenomenon of alternations in 

 cleavage we may now consider the significance of the peculiar 

 features of radial and bilateral cleavages. 



(i) Significance of Orthoradial Cleavages. — The most re- 

 markable thing concerning orthoradial cleavage is that it does 

 not conform to the principle of minimal contact surfaces. So 

 far as I can recall, all eggs which are said to exhibit this form 

 of cleavage, e.g., Amphioxus, echinoderms, Scyandra, do not 

 exhibit a compact form, but consist of a number of blasto- 

 meres loosely piled together, and generally with a large seg- 

 mentation cavity between them. During the early stages of 

 cleavage these blastomeres are individualized to such an extent 

 that they are globular and are not closely pressed against 

 their neighbors. There are therefore no rotations of the blas- 

 tomeres, and consequently no polar furrows or pressure sur- 

 faces. The compactness of the &gg is sacrificed to the inde- 

 pendence of the segment spheres. It may be worth while to 

 remark in passing that this independence in form is generally 

 associated with a great amount of independence in function, as 

 experiment has demonstrated in the case of echinoderms and 

 Amphioxus. Sooner or later these independent blastomeres 



