Pilid'mra of Certain Nemerteans. 241 



widely separated from each other. Indeed, they do not usually 

 come even in contact at first (Fig, 4). Each lies above, slightly out- 

 side, and to the right (following the hands of a watch) of its sister 

 cell. It is interesting to note how these cells, at the time of their 

 origin only indirectly connected, enter into the most intimate union 

 in the later stages. 



As the upper cells draw together they move so far to the right 

 that they finally occupy positions almost, though not quite, in the 

 intervals of the cells below (Fig, 6), This drawing together and 

 rotation of the blastomeres of a segmenting egg is doubtless mainly 

 due to the general physical property of surface tension. As stated 

 by Wilson (34), the spiral type of cleavage " owes its peculiarities 

 entirely to mechanical conditions, the blastomeres assuming the posi- 

 tion of greatest economy of space, precisely like soap-bubbles or 

 other elastic bodies." It is not unlikely that these purely physical 

 conditions are aided by a slight mutual attraction exerted by the 

 living protoplasm of the blastomeres. 



Seen from the side (Fig. 5) the upper cells in front lie above and 

 to the left of their sister cells. Even in the eight-celled stage there 

 is a perceptible segmentation cavity in the center. 



At the fourth cleavage the upper four cells of the eight-celled 

 stage divide obliquely downward and to the right, while the lower 

 four divide obliquely upward and to the left. This cleavage is there- 

 fore typically left-handed. The sixteen cells become arranged in 

 four zones (Fig. 7), the cells of each zone forming a nearly perfect 

 square, and alternating with those above and below. 



The fifth cleavage is right-handed, the sixth left-handed, and so 

 on, conforming to the regular spiral type. In the 16-cell stage the 

 segmentation cavity is more than equal in diameter to one of the 

 blastomeres, and in the later stages increases rapidly in size. In the 

 sixth cleavage some of the thirty-two cells divide earlier than others, 

 so that we have several distinct stages between the 32- and the 

 64-celled stage, 



Grastrulation. 



About nine hours after fertilization cilia appear on the outer surface 

 of all the cells, and the embryo begins to swim. With the appear- 

 ance of cilia a marked differentiation of the cells on the opposite sides 

 of the blastula is noticeable, and the embryo rotates about a single 

 axis. The rate of rotation is about two or three revolutions per 

 second. The cells of the upper pole (corresponding with the side of 



Trans. Conn. Acad., Vol. X. August, 1899. 



16 



