OASTLE: EMBRYOLOGY OF OIONA INTESTINALIS. 233 
Figures 36 and 37 we see in contact cells which we know, from an ex- 
amination of Figure 42, eventually become the anterior ventral and 
posterior dorsal portions of the embryo. 
We have now followed the cleavage cell by cell to the 24-cell stage. 
We have seen that cleavage is from the very beginning bilateral, and 
progresses in a very definite manner and at a very definite rate. "This 
we shall find is true in the further development of the egg, even until 
the complete closure of the blastopore. Wilson ('94) observed that the 
cleavage of Amphioxus showed all gradations between a. perfectly radial, 
a bilateral, and even a spiral form; and he raised a query whether the 
same might not be found to be true for Ascidians. In Ciona at least this 
does not seem to be the case. I have never observed an instance of 
deviation from the regular mode of cleavage described in the foregoing 
paper, unless one so construes the occasional very slight difference in the 
time of cleavage of the cells of the two hemispheres in passing from the 
8-cell stage, a matter to which allusion was made on page 232. No 
rotation of the cells of one hemisphere over those of the other even in 
the slightest degree bas ever been observed. In having a perfectly 
definite and stereotyped manner of cleavage, the ascidian egg resembles 
more closely the egg of Annelids, Mollusks, and the great majority of 
Invertebrates, than it does that of Amphioxus and the Vertebrates, not- 
withstanding that the emd product of cleavage shows unmistakably the 
now generally admitted closer affinity of Tunicates with the latter group 
of animals. 
It remains to call attention to some of the internal phenomena accom- 
panying the early cleavage stages. The first, cleavage spindle arises, as 
has been stated, not far from the.centre of the egg. (See Plate III. 
Fig. 14.) As its first cleavage is nearing completion, however, the 
attraction spheres and nuclei begin to move toward the dorsal surface o 
the egg, away from its more richly protoplasmic (animal) pole, from 
which the plane of separation cuts in more rapidly. (See Plate III. 
Fig. 15.) The attraction sphere of each blastomere grows more diffuse 
as the nuclei pass into a resting condition ; it then elongates in a hori- 
zontal direction and parallel to the first plane of cleavage, and finally 
divides. "The parts separate and the nucleus moves out to a position be- 
tween them. (See Plate III. Fig. 16.) By this time the attraction 
spheres and nuclei unmistakably lie closer to the dorsal (maturation) 
surface of the egg. (Plate IIT. Fig. 16 ; cf. Plate IV. Figs. 20, 21, and 
Plate V. Figs. 27-29; also Van Beneden et Julin's ['84] Figs. 2 and 46, 
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VOL, XXVII. — NO. 7. 
