22 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY, 



(Plate 2, Fig. 8) shows that the cleavage planes have taken the posi- 

 tions foreshadowed by the arrangement of the spindles. The plane 

 separating C^ from D^ lies to the right of the plane separating A^ from 

 B^ ; the corresponding furrows on the surface are nearer together on 

 the dorsal than on the ventral side. The blastomeres resulting from 

 the division oi AB' are equal, whereas CD' divides very imequally. 

 The right derivative (C^) is much smaller than the left {D^), and is of 

 approximately the same size as A^ and B^. The blastomeres B^ and 

 JD^ are in contact along the whole distance from the dorsal to the ven- 

 tral surface of the eg^^, while A^ and (7^ do not touch each other at all. 

 The polar cell lies either at the junction of B^, C% and I^, as shown in 

 Figure 8, or sometimes at the junction of A^, B^, and B^. The egg is 

 now markedly unsymmetrical. 



It is evident from the above description that this cleavage may be 

 considered as belonging to the so called spiral type. Since the left end 

 of the spindle is in each cell the higher, the cleavage is a left spiral, 

 like the corresponding cleavage in Discocelis, Nereis, Limax, and indeed 

 all forms with spiral cleavage except in the reversed cleavage of certain 

 mollusks. This fact is striking, since the succeeding cleavages in 

 Asplanchna do not belong to the spiral type. 



The relation between the axes of the embryo in later stages and the 

 first two cleavage planes is as follows. The first furrow separates an 

 anterior from a larger posterior portion, but the plane of separation of 

 the parts bears no simple relation to the axes of the later embryo. 

 (Compai-e Figure 8 with Figure 75, Plate 9, in which the parts derived 

 from the first four cells are colored in the same manner as their parent 

 cells in Figure 8, and note the great shifting.) The later sagittal plane 

 of the embrj'o is coincident with a plane passing through the animal pole 

 and the longest axis of the e^g ; that is, through the plane separating A^ 

 from B^ (Fig. 8), and dividing the larger blastomere D^ into two un- 

 equal parts. The second cleavage plane therefore divides the right side 

 from the left in the anterior part of the egg ; but in the posterior part it 

 lies entirely in the right side. It is not until the seventh cleavage that 

 the division into symmetrical right and left halves takes place on the 

 posterior side (Plate 7, Fig. 58) ; indeed, certain cells containing material 

 for both sides of the e^g remain undivided till even a later stage. 



Third Cleavage. 

 Immediately after the second cleavage, the aster in each of the four 

 cells produced begins to extend dorso-ventrally, at right angles to the 



