124 BULLETIN OF THE 



The cleavage plane, 1 dpi, whicti divides the ovum into two segments, 

 may be called a meridional plane. It is the first cleavage plane. In 

 the formation of the 4-cell stage the two segments already formed are 

 divided by a plane at right angles to this, and the mode of division in 

 the two is identical. The division of the two cells which form the 

 2-cell stage begins by a slight constriction, girting the spheres which 

 later bisects them, forming four smaller nucleated spheres or blas- 

 tomeres, all of the same size. There is no 3-cell stage in this kind of 

 cleavage. The second plane of cleavage divides both cells of the 2-cell 

 stage. 



The formation of the second cleavage plane will thus be seen to differ 

 from that of Asterina, as described and figured by Ludwig.* In Asterina 

 the two cells of the 2-cell stage are of unequal size. The smaller of 

 these divides first, so that we have a 3-cell stage, fig. 2 (op. cit.). 

 In Echinarachnius both the cells were observed to divide at the same 

 time and form the 4-cell stage. The cleavage plane which forms the 

 4-cell stage (2nd cleavage plane) in Echinarachnius is at right angles to 

 the first, and identical in its position in each cell of the 2-cell stage. 

 Ludwig, p. 6, op. cit., says of Asterina : " Die Theilungsebene der 

 beiden Zellen II. (larger cell of 2-cell stage) ist aber nicht etwa die 

 auf die Zelle II. iibergreifende Theilungsebene der Zellen I. (smaller 

 cell of the 2-cell stage), sondern bildet mit letzterer, so wie auch mit 

 der Theilungsebene der beiden ersten Furchungskugeln einen rechten 

 "Winkel." Three cells were observed abnormally formed in the ovum of 

 of Echinarachnius, and their mode of formation is traced below. 



In Strongylocentrotus, according to A. Agas8iz,t after the yolk sepa- 

 rates from the inner wall of the outer envelope, it is slightly depressed 

 on one side, and a similar change soon after occurs on the opposite pole. 

 After these depressions in the poles of the yolk of Strongylocentrotus 

 occur, a slit is formed, according to A. Agassiz, which divides the egg 

 into two large elliptical masses. 



In the egg of Echinarachnius in normal cases a constriction was 

 observed, PI. II. fig. 2, girting the yolk, similar to fig. 23, p. 709, of 

 the work last mentioned.t This constriction deepens uniformly on all 

 sides until the 2-cell stage is formed. In several eggs of Echinarach- 

 nius, PI. III. figs. 1, 2, 3, the 2-cell stage is formed in another way. 



* Entwickelungsgeschichte der Asterina gibbosa, Forbes. Zeit. f. Wi&s. Zool., 

 XXXVII. pp. 6, 7. 



t Revision of the Echini, p. 710. 



