48 SEGMENTATION. 
we see that it is only a variant of the /endency for offspring to resemble 
their parents that the oztogeny (of an individual) ¢ends to be a rapid re- 
capitulation of the phylogeny (of the group). This is termed the primary 
Biogenetic Law of Recapitulation. The tendency can only be turned 
into an actual fact in those (practically non-existent) cases in which the 
ontogenetic migration exactly recapitulates the phylogenetic 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 /arva 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 canogenetic 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 coenogenetic 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 4/astomeres, 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 a/ecithal eggs. 
In the majority of developments, however, the egg 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 
