ii8 SEGMENTATION [CH. 



partial, as happens when a large amount of yolk is present, 

 at least in the case of telo-lecithal eggs such as those of 

 Vertebrates and Cephalopods, in which the greater part of 

 the protoplasm is at one pole and in which the cleavage 

 furrows divide the polar cap of protoplasm but do not ex- 

 tend far into the yolk. The problem in centro-lecithal eggs, 

 characteristic of Arthropods, is somewhat different, and 

 will be considered later. 



In many eggs in which the first cleavage plane has a 

 definite relation to the position of parts in the future em- 

 bryo it is undoubtedly true that this orientation of the 

 embryo depends on the pre-existing structure or polarity of 

 the egg, and in these the position of the first cleavage plane 

 is also determined by the same causes. In a number of eggs, 

 however, there is no such complete predetermination. It is 

 maintained by DRIESCH and others that eggs of this type are 

 equi-potential systems, that is to say, they are not differenti- 

 ated before fertilisation into regions of different potentiality, 

 and any part of the egg is capable of giving rise to any part 

 of the embryo. It is doubtful whether any egg exists which 

 is not to some extent differentiated into different regions, 

 but experiments in separating blastomeres after the first or 

 second cleavage division show that in a number of eggs each 

 of the first four blastomeres can give rise to a perfect larva, 

 and that therefore the differentiation of parts in the embryo 

 cannot be due solely to pre-existing arrangements of "organ- 

 forming" substances in the egg. Eggs of this type bring to 

 ouTk notice in a striking manner one of the fundamental 

 problems of biology, the problem of the differentiation by 

 division of a cell in which the parts are equipotential into 

 an embryo in which the parts are conspicuously different. 

 Most commonly, under natural conditions, the first step of 

 this process is the division of the egg into two blastomeres 



