CLEAVAGE 247 



symmetry of the egg appears to be radial or rotatorial before 

 fertilization and is converted into a bilaterally symmetrical 

 structure by the entrance of the spermatozoon, the entrance 

 path of which marks the plane of symmetry of the egg and 

 developing organism, and determines the location of the first 

 cleavage. It is quite possible, though hardly demonstrated 

 as yet, that even in such cases there is really an invisible bilateral 

 structure of the ovum which underlies the radial symmetry 

 and really determines the point at which the sperm shall enter. 

 In such a case the entrance path of the spermatozoon would 

 itself be predetermined and could not be regarded as a primary 

 factor in fixing the position of the first cleavage. This would 

 obviously be the case in many of those eggs possessing micro- 

 pyles. But in some eggs whose cleavage is indeterminate, even 

 though they possess micropyles (Teleosts), there seems to be 

 no regularity in the position of the first cleavage plane and no 

 correspondence between this and any morphological char- 

 acteristic of either ovum or adult. 



The second cleavage, usually at right angles to the first, 

 ordinarily corresponds with the median transverse axis of the 

 egg, embryo, and adult. The third cleavage is usually hori- 

 zontal and separates animal and vegetal poles and corresponds 

 most frequently with the separation, in the embryo and adult, 

 of the more active animal, and less active vegetative tissues. 



The facts that in all eggs of a given species or genus cleavage 

 occurs according to a definite pattern, and that there may be 

 an exact relation between the individual blastomeres of the 

 cleaving ovum and the tissues and organs of the later organism, 

 make it possible to speak of the "cell lineage" (Wilson) of an 

 organism. In forms with determinate cleavage it becomes 

 possible to identify, even in a comparatively late embryonic 

 stage, various groups of cells as the real lineal descendants of 

 certain individual cells of the earlier cleavage group. In other 

 words, it is in such cases possible to trace the structures of the 

 embryo and adult back to single cells or parts of cells. 



In order to illustrate the nature of the facts of cell lineage, 

 and the completeness and exactness of the correspondence be- 



