206 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
the process of division of the yelk into smaller and 
smaller portions goes on. 
In a great many animals, the splitting-up into blasto- 
meres is effected in such a manner that the yelk is, at 
first, divided into equal, or nearly equal, masses; that 
each of these again divides into two; and that the number 
of blastomeres thus increases in geometrical progression 
until the entire yelk is converted into a mulberry-like 
body, termed a morula, made up of a great number 
of small blastomeres or nucleated cells. The whole 
organism is subsequently built up by the multiplication, 
the change of position, and the metamorphosis of these 
products of yelk division. 
In such a case as this, yelk division is said to be 
complete. An unessential modification of complete yelk 
division is seen when, at an early period, the blastomeres 
produced by division are of unequal sizes; or when they 
become unequal in consequence of division taking place 
much more rapidly in one set than in another. 
In many animals, especially those which have large 
ova, the inequality of division is pushed so far that only 
a portion of the yelk is affected by the process of fission, 
while the rest serves merely as food-yelk, for nutriment 
to the blastomeres thus produced. Over a greater or 
less extent of the surface of the egg, the protoplasmic 
substance of the yelk segregates itself from the rest, 
and, constituting a germinal layer, breaks up into the 
blastomeres, which multiply at the expense of the food- 
