76 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



statement is of course utterly without significance. The real question 

 is, What determines the point where it divides? Why in Asplanchna 

 does the middle of the sphere of action in the fourth and seventh 

 cleavages uf the Lirge ventral entoderm cell lie in such a position (Fig. 

 16, Plate 2, and Fig. Go, Plate 8) as to divide the cell into parts which 

 are only slightly unequal, while in the intervening fifth and sixth 

 cleavages (Fig. 35, Plate 5, and Fig. 49, Plate 6) the middle of the 

 sphere of action is at the periphery of the cell 1 "^Vhy at the first 

 cleavage of the frog's egg is the middle of the sphere of action in the 

 centre of the mass of formative protoplasm, whereas at the divisions 

 immediately preceding — the formation of the polar cells — it is at the 

 periphery of the egg ] 



In regard to the question what determines the comparative rate of 

 cleavage the case is similar. In view of the recent discussion of these 

 two questions — the inequality of cleavage and the I'ate of cleavage — in 

 the works of Kofoid ('94, p. 196), McMurrich ('95), Lillie ('95, p. 45), 

 zur Strassen ('95), Ziegler ('95), and others, it seems scarcely worth 

 while to insist upon the fact that the rate of cleavage and the equality 

 or inequality of the products are related to the future morphogenetic 

 processes, and show in many cases no relation whatever to accumulations 

 of yolk. Yet the fo called laws, according to which these matters are 

 determined by the distribution of yolk, are repeated in 0. Hert wig's 

 text-book on the cell (Hertwig, '93, pp. 174 and 180), and reaffirmed in 

 the latest edition of his treatise on embryology (Hertwig, '96, pp. 67 

 and 84). 



In Asplanchna the main facts in regard to the sequence of cleavage 

 are as follows. 



(1) The order of cleavage is, within very narrow limits, constant. If 

 a number of eggs are taken showing successive stages of the division of 

 any given cell, the series will show corresponding successive stages in 

 the nuclear history of the other cells. 



(2) There is a typical order for similar cells of different quadrants of 

 the same egg. This order is D, C, B, A. 



(3) There is a marked general tendency for larger cells to divide 

 first. In every case where two cells of the same origin are of different 

 size, the larger divides before the smaller. 



This is a very general phenomenon, as has been repeatedly pointed 

 out of late, even in cases where the larger cell contains the greater 

 amount of yolk. The "law" recently formulated by Hertwig ('96, 

 p. 67), that " die Schnelligkeit der Fui'chung proportional ist der Con- 



