CASTLE : EMBKYOLOGY OF CIOXA INTESTINALIS. 233 



Figures 3G 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. ^o 

 rotation of the cells of one hemisphere over those of the other even in 

 the slightest degree has 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 end 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 towai'd the dorsal surface o 

 the egg, away from its more richly protoplasmic (animal) pole, from 

 whicli the plane of separation cuts in moi'e rapidly. (See Plate III. 

 Fig. 15.) The attraction sphere of each hlastomere grows more diffuse 

 as the nuclei pass into a resting condition ; it then elongates in a hori- 

 zontal direction and parallel to the first i)lane of cleavage, and finally 

 divides. The parts separate and the nucleus moves out to a position be- 

 tween them. (See Plate III. Fig. IG.) By this time the attraction 

 spheres and nuclei unmistakably lie closer to the dorsal (maturation) 

 surface of the egg. (Plate III. 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, 



vol.. XXVII. — NO. 7. 3 



