CLEAVAGE IN THE EGG OF CEREBRATULÜS. 



9 



VIII. Early Cleavage Stages of Trefoil Eggs 

 (C. lacteus and C tnarf/inatiis). 



Trefoil eggs^ (Fig. 19) were isolated from the normal eggs, 

 and their mode of cleavage was studied singly. This observation 

 was made first in 1901 and was repeated both in 1905 and 1906. 



Fig. III. {Oerebratiiliis m'irgina/us 12-16, 18 ; C. lacteus 17). 



12-18, early cleavage stages of the eggs treated with Ca-free sea-water. 

 12, 4-celI stage ; 1.3, beginning of the third cleavage ; 14, 8-cell stage, 

 with blastomeres arranged in two rows ; 15, 8-cell stage, with blastoineres 

 arranged in a ring ; 16, beginning of the fourth cleavage ; 17, 16-celI stage ; 

 18, a stage preceding the 32-cell stage, seen from the vegital pole. Upper 

 cells are stippled. 



1 Trefoil egg = " Sioiiiltandreier." It may be mentioned that there are two kinds of 

 trefoil esg-i: (a) the one dne to three vertical planes, and (b) the other due to one vertical 

 and two transverse cleavage planes. These two cases can be very readily distinguished by 

 the position of polocytes. In this section the former kind alone is meant by trefoil egg, 

 since the hitter is of little value for the present purpose. The "Simultanvierer" is very 

 seldom met with in Cerebrafuius. 



