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 

 fii'st, 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 subsequent!}^ built up by the multiplication, 

 the change of position, and the metamorphosis of these 

 products of 3'elk 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- 



