298 Charles Zeleny. 



soon fades away and within half an hour the only sign of it is a 

 clear area which has collected near the pole opposite the pro- 

 tuberance. In this clear area is the spindle of the first polar di- 

 vision. The egg remains in this condition unless fertilized. In 

 the latter case the first and the second polar bodies are formed in 

 succession (Figs. 6, 7 and 9). The cell then elongates, exter- 

 nally a cleavage furrow appears in the vicinity of the polar bodies 

 and later another one at the opposite pole. These constrict the 

 egg Into two equal parts. The second furrow, at right angles to 

 the first, also passes through the polar bodies, and the two to- 

 gether divide the egg into four equal parts with no cross furrow 

 or very little indication of one. After the third cleavage, dexio- 

 tropic as usual, the cells of the upper quartet are distinctly larger 

 than those of the lower quartet. The cleavage goes on as a per- 

 fect Illustration of the spiral type. The cells of the eight-cell 

 stage give out smaller cells by leiotropic cleavages, taking place 

 simultaneously. The further divisions of the cells of the six- 

 teen-cell stage thus formed are not simultaneous. As in C. lac tens,- 

 the cell of the Intermediate group in each quadrant which had 

 been given off by the basal cell of that quadrant lags behind the 

 others, so that there is a distinct twenty-eight-cell stage before the 

 tardy cells divide to form the thirty-two-cell stage. The method 

 of cleavage as here described is very constant for normal whole 

 eggs, variations being extremely rare. 



The character of the normal pilidium is too well known to need 

 any description here. The essential features of the early cleav- 

 ages are given in Figure i, p. 296, and three stages of the larval 

 development are shown In Figure 2, p. 300. 



4. The Experimental Results. 



0. Introduction. The experiments are described here in turn 

 according to the period at which the operation was performed. 

 Ten such periods are recognized: 



1. Unfertilized egg. 



2. Fertilization to complete separation of the first polar body. 



3. First polar body to complete separation of the second polar 

 body. 



