240 TK 7?. Coe — Development of the 



narrow bridge of protoplasm. Later they become closely pressed 

 together (Fig. 1). 



The spindles of the second cleavage occupy, as is usual, positions 

 nearly at right angles to that of the first, and the second division is 

 likewise vertical. The four resulting blastomeres are again almost 

 exactly equal in size. They are also apparently similar in regard to 

 their constituent protoplasm, for each of the four cells may be con- 

 sidered as being made up of one quarter of the finely granular 

 protoplasm which was collected at the animal pole during the pro- 

 cess of fertilization, and of one quarter of the deutoplasm which 

 segregated towards the vegetal pole at the same time. There is no 

 reason for believing, however, that the cells are actually alike as 

 regards their ultimate constitution, for, as we shall see, it is proba- 

 ble that one of the cells has quite a different sphere of activity than 

 any of the others. The second cleavage does not take place exactly 

 at right angles to the first, so that two of the cells lie at a slightly 

 higher level than the other two. This is one of the first indications 

 of the spiral nature of the cleavage, M^hich becomes so conspicuous 

 in the later stages. In the Mollusk Crepidula Conklin (9) makes 

 the very interesting observation that the obliquity of the cleavage is 

 manifest even before the first cleavage. 



Immediately after the completion of the second division the result- 

 ing blastomeres are almost perfectly spherical in form, so that they 

 touch only on very small surfaces (Fig. 2, PI. xxxi). Eight or ten 

 minutes later, however, they have drawn so closely together that 

 the surfaces of contact almost meet in the center, and leave only a 

 very small segmentation cavity (Fig. 3). It sometimes happens 

 that this cavity is entirely obliterated above, in which case the cells 

 B and I> commonly come in direct contact and close up the space, 

 while A and C do not meet at all. The four cells form a nearly 

 perfect square, one side of which is about equal to the diameter of 

 the unsegmented egg. 



When the spindles of the third cleavage are forming, each one is 

 directed upward and somewhat to the right, showing that the third 

 division is to be right-handed. After the separation of the eight 

 blastomeres all are again of nearly spherical shape. The upper four 

 are slightly, though distinctly, larger than the others,* and are more 



* Id C. leidyi and in C. lacteus such is likewise the case. In Tetrastemma, on the 

 other hand, the upper four cells of the eight-celled stage are described by Lebedinsk.y 

 (20) 33 being smaller than the lower four. In other nemerteans all of the eight cells 

 are said to be of equal size. 



