294 



MEAD. 



[Vol. XIII. 



The direction of cleavage. — Mechanical principles which are 

 apparently sufficient to explain the direction of cleavage in 

 one instance are totally inadequate in others. Thus, when 

 we have decided that certain factors determine the hori- 

 zontal direction of the first cleavage spindle, for example, the 

 principle of equal or least resistance due to the segregation 

 of yolk, these factors must be considerably modified when called 

 upon to explain the vertical position of the two maturation 



spindles ; for the con- 

 ^- B. C. ditions, segregation 



of yolk, etc., are the 

 same.* 



The appearance of 

 the peculiar lobe in 

 ChcetoJ)tenis, simul- 

 taneously with the 

 first cleavage, seems 

 to indicate that the 

 cleavage does not 

 take place in the 

 direction of least re- 

 sistance, else it would coincide with the axis of the lobe, as in 

 Loeb's "^ experiments on sea-urchin eggs. 



In this type of cleavage the spindles of the two blastomeres 

 are inclined to each other, and almost always in the same direc- 

 tion. Why is this .' And why does Physa form an exception 

 to the rule.-* 



In Lepidonotics, Amphitrite, and Clymenella from the division 

 of the first two blastomeres up to the "ideal" 64-cell stage 

 the regular alternation in the direction of cleavage accords with 

 the "law" that successive cleavage planes tend to lie at right 

 angles. f After the ideal 64-cell stage many of the cells divide 



2® J. LoEB: On Some Facts and Principles of Physiological Morphology. 

 Biol. Lecttires. Woods Holl, 1893. 



* Watase.^^ in his "Studies on Cephalopods,"yi7?/r«. of Morph., vol. IV, no. 3, 

 figures an egg of Loligo in which the blastoderm is developing on the side instead of 

 on the end of the egg. The pattern of the cleavage is the same as in the normal 

 egg, though the conditions must be decidedly different. 



t In the retarded cleavage on the upper hemisphere of Umbrella there 

 are certain divisions which do not agree in direction with the corresponding 



Fig. XXIII. — A , Lepidonoius : /, apical cells ; //, their prod- 

 ucts. B, Lepidonotus : I, vegetative-pole cells ; //, their 

 products. C, Discoccelus : /, vegetative-pole cells ; //, their 

 products. 



