48 SEGMENTATION. 



we see that it is only a variant of the tendency for offspring to resemble 

 their parents that the ontogeny (of an individual) tends to be a rapid re- 

 capitulation of the phytogeny (of the group). This is termed the primary 

 Biogenetic Law of Recapiiutation. The tendency can only be turned 

 into an actual fact in those (practically non-existent) cases in which the 

 ontogenetic migration exactly recapitulates Xho. phyloge^tetic migration. 



From these considerations it is easy to see that an embryonic de- 

 velopment never conforms to the law of recapitulation, for the environ- 

 ment of the embryo is at every stage quite different to that of the 

 corresponding phyletic stage. 



A purely larval development may, in the impossible event of an 

 exactly similar sequence of environments (or migration), conform to the 

 law. An approximation only to this ideal can be attained and the want 

 of conformity results in this important truth, that a larva at a certain 

 stage of its existence has a given number of its characters which are 

 palingenetic or resembling similar stages in the phylogeny, and others 

 which are ccenogenetic or developed in conformity with the new environ- 

 ment which has been adopted at that stage. (The palingenetic 

 characters owe their presence to heredity, the ccenogenetic to adapta- 

 tion, using these terms as applied to the race, not to the individual. ) In 

 the embryonic type the environment is so fundamentally changed that 

 the ccenogenetic characters usually outweigh the palingenetic and many 

 of the latter are completely obliterated. 



Segmentation. — In larval and embryonic forms alike 

 there is the same necessity for the conversion of the uni- 

 cellular ovum into a multicellular organism. This is 

 attained by rapid cell-divisions or segmentation of the 

 ovum. In some embryonic types the multiplication is at 

 first confined to the nuclei, the cell-walls only appearing 

 later, but this is clearly only a retarded instance of seg- 

 mentation. 



Types of Segmentation. — In many larval types the 

 ovum segments by a series of binary fissions into a hollow 

 (or occasionally solid) ball or sphere of cells. The 

 segments are termed blastomeres^ are produced in multiples 

 of two, and are equal. This type is called total equal 

 segmentation^ and occurs in eggs with little or no yolk, 

 usually termed alecithal eggs. 



In the majority of developments, however, the ^gg has 

 an endowment of nutritive material from the parent, called 

 yolk, which is the beginning of the embryonic type. 

 This yolk enables the young form to dispense with the 

 necessity for ingestion of food. At the same time it affects 

 the segmentation. If the yolk were evenly distributed 

 throughout the egg, and not too abundantly, the only effect 



