CLEAVAGE AND DEVELOPMENTAL PATTERN 591 



because the membrane tends to return to its original spherical form. In 

 absence of the membrane, removal of pressure results in divisions vertical 

 to the previous direction, so that the plate becomes two cells thick. Eggs 

 with cleavage pattern thus altered may develop into normal plutei.^^ 

 From these modifications of cleavage pattern Driesch concluded that 

 blastomeres of the sea-urchin embryo are equivalent and can be mixed in 

 any order without preventing normal development. A critique by Braem 

 (1893) showed this conclusion to be unnecessary, and these experiments 

 are now generally regarded merely as evidence for independence of early 

 cleavage pattern and developmental pattern. If developmental pattern 

 is a general or a cortical gradient pattern, these changes in cleavage pat- 

 tern may not affect it at all, and even a regionally specific pattern might 

 persist essentially unaltered by the changes in cleavage. 



Even in nemerteans, annelids, and mollusks — forms with spiral cleav- 

 age — ^pattern of early cleavage can, to some extent, be dissociated from 

 developmental pattern. In eggs of species of all three phyla under pres- 

 sure, cleavage spindles tend to lie in the greatest diarheters of the eggs, that 

 is, vertical to direction of pressure, and plates of eight or sixteen cells may 

 result, but the egg polarity is also a factor in determining cleavage pattern 

 in these experiments. For example, in eggs of the nemertean Cerebratulus, 

 with pressure at right angles to the polar axis, the first cleavage plane 

 passes through polar and pressure axis; the second, at right angles to the 

 first, also passes through the pressure axis, that is, it is equatorial with 

 respect to the polar axis, instead of meridional like the second normal 

 cleavage. If pressure is removed at this stage, the third cleavage planes 

 are meridional, like the second of normal development, and further de- 

 velopment is normal. Plates of eight or sixteen cells may also result from 

 pressure, but pilidia developing from these are more or less defective or 

 abnormal, sometimes partial duplications involving the apical organ 

 (Yatsu, 1910a, b). According to Dederer (19 10), polarity of the Cerebratu- 

 lus egg can be altered by pressure; but since no other experimenter has 

 observed such alteration in forms with spiral cleavage, this may be an 

 error. 



Polar-body position is not altered in the egg of the mollusk Cumingia 

 by pressure in any direction, and the first cleavage plane usually passes 

 through the polar axis, though with pressure at right angles to this axis, it 

 is sometimes obhque or equatorial (Browne, 1910). With sufficient pres- 

 sure in the direction of the egg axis of some other forms with spiral cleav- 



=' Driesch, 1892; Morgan, 1894; Ziegler, 1894. 



