252 SOME RECENT 



the embryologist will find the clue he has so 

 long waited for, which will enable him to distinguish 

 the real from the spurious, the ancestral from the 

 acquired characters. 



The earliest stage of development in every Meta- 

 zoon is the act of cell-division, or segmentation as 

 it is commonly termed, by which the single cell, or 

 egg, becomes divided into a number of cells or 

 blastomeres. The details of the process vary greatly 

 in the eggs of different animals, and are affected 

 more particularly by the amount of food yolk, or 

 deutoplasm, present in the egg; this food yolk 

 offering mechanical hindrance to the division of the 

 egg. In each species or genus, or even in larger 

 groups, the mode of segmentation was formerly 

 assumed to be constant, but the more recent re- 

 searches have shown that this is by no means 

 always the case. 



In 1878, Kleinenberg noticed that the eggs of an 

 earth-worm, Lumbricus trapezoides, which he was 

 investigating, showed considerable individual differ- 

 ences in the mode and order in which the successive 

 cell-divisions were effected ; an observation which 

 was afterwards confirmed by Wilson, and extended 

 to other species of earthworms. In 1884, while 

 studying the development of Renilla, one of the Pen- 

 natulida or Sea- Pens, Wilson found an extraordinary 

 range of variation in the mode of segmentation of 

 eggs, even of the same brood. In some cases the 

 egg divided into two blastomeres in the normal 

 manner, each of these in its turn again dividing ; in 

 other cases however the egg divided at once into 



