282 



CELL-DIVISION AND DEVELOPMENT 



into apparent confusion, first by the discovery of Clapp ('91), Jordan, 

 and Kycleshymer ('94) that in some cases there seems to be no con- 

 stant relation whatever between the early cleavage-planes and the 

 adult axes, even in the same species (teleosts, urodeles) ; and even in 

 the frog Hertwig showed that the relation described by Newport and 

 Roux is not invariable. Driesch finally demonstrated that the direc- 

 tion of the early cleavage-planes might be artificially modified by 

 pressure without perceptibly affecting the end-result (cf. p. 309). 



These facts prove that the promorphology of the early cleavage- 

 forms can have no fundamental significance. Nevertheless, they are 

 of the highest interest and importance ; for the fact that the forma- 



P 



Fig. 127. Bilateral cleavage of the squid's egg. [WATASK.] 



A. Eight-cell stage. B. The fifth cleavage in progress. The first cleavage (a-/) coincides 

 with the future median plane ; the second (l-r) is transverse. 



tive forces by which development is determined may or may not 

 coincide with those controlling the cleavage, gives us some hope of 

 disentangling the complicated factors of development through a com- 

 parative study of the different forms. 



(c) Other Promorphological Characters of the Ovum. Besides the 

 polarity of the ovum, which is the most constant and clearly marked 

 of its promorphological features, we are often able to discover other 

 characters that more or less clearly foreshadow the later develop- 

 ment. One of the most interesting and clearly marked of these is 

 the bilateral symmetry of the ovum in bilateral animals, which is 

 sometimes so clearly marked that the exact position of the embryo 

 may be predicted in the unfertilized egg, sometimes even before it is 

 laid. This is the case, for example, in the cephalopod egg, as shown 



