AX INTRODUCTION TO THE STUDY OF GROWTH. 



the eggs of the Lancelet, Amphioxus. 1 In this simple 

 fish it is possible to separate from one another the first 

 two cells formed by the division of the egg. The cells 

 thus obtained continue to develop and go through a 

 number of stages characteristic of the normal egg, but 

 the larva is only half as large as the normal one, its 

 component cells being reduced in size. This larva dies 

 young, and whether it could be made to develop further 

 is left for future experiment to decide. When the egg 

 has already formed four cells, it is still possible in the 

 same manner to separate one of these, which will also 



FIG. 5. Showing the egg of Amphioxus in the 

 one (A), two (B), and four cell (C) stages and 

 below the corresponding larva;, a, b, c developed 

 in each case from a single cell. The larva be- 

 comes smaller as the fraction of the egg becomes 

 less, X 135 diam. (Wilson.) 



develop alone, and give rise to a complete larva, save that 

 it is only one-quarter the normal size. This is still more 

 difficult to rear (Fig. 5). During the past year it has 

 been shown (Morgan) that fragments of the unsegmcnted 

 egg of the sea urchin may, when they contain both the 

 male and female pronuclei, develop gastulx, and this 

 occurs even if nucleated fragments have but one-fiftieth 



1 Wilson, Journal of Morphology, 1893. 



