4 BULLETIN: MUSEUM OF COMPARATHT: ZOOLOGY. 



PART FIRST. — DEVELOPMENTAL MECHANICS. 

 I. Statement of Problems. 



We shall deal in the following pages with (1) cleavage, (2) gastrula- 

 tiou, and (3) the relation of these to each other. 



1. Cleavage. 



It will be necessary, in studying the cleavage and the factors deter- 

 mining it, to enter into minute details as to the movements of asters, 

 the form and dimensions of cells, and other similar matters : the effort 

 of following this, in itself somewhat laborious, description will be much 

 lightened by holding in mind the problems iipon which it bears. I 

 shall tlierefore give first a statement of the main theories which have 

 been advanced as to the determining factors in cell division. 



Cell division presents three aspects, in each of which its nature is in 

 some way determined. (A) As to the direction of cleavage : the posi- 

 tion in which the new septum is to appear. Since this bears a definite 

 relation, in general, to the position of the spindle leading to the cleav- 

 age, we may speak of this aspect as the determination of the direction 

 of the spindle. (B) As to the relative size of the two products; 

 whether the division is equal or unequal. (C) As to the relative time 

 of division, or the interval between successive cleavages. 



Besides these, we have (D) the question of the qualitative nature of 

 cleavage. Are all the cells that are produced of similar structural and 

 material character, or is cleavage accompanied by qualitative diffei'entia- 

 tion of the blastomeres, — either as a result of qualitative karyokinesis 

 or otherwise 1 



A. Theories as to the Factors determining the Direction of the Spindle 

 and the Position of the new Cell Wall. 



The theories as to the factors determining the direction of cleavage 

 are numerous, and have been much discussed of late. General reviews 

 of these theories will be found in Driesch ('92, p. 2G), Braem ('94, 

 p. 340), Ziegler ("94, p. 136), and McMurrich ('95). I shall give here 

 as brief and precise a statement of each theory as possible, first in my 

 own words, then, so for as practicable, in a quotation from the author. 



(1) BertholcFs principle of least surfaces. — Berthold ('86) holds that 

 the form and relative position of cells, and as a consequence their direc- 



