540 William A. Hilton. 



formed from the four-cell stage bj clefts whicli branch off from the 

 earlier cleavages, this represents a six-cell, and in this egg at this 

 time the first cleavage plane has nearly encircled the egg and now for 

 the first time there is an indication of the first segmentation plane 

 from the vegetative pole (Fig. 15). Another egg of a similar stage 

 of development is shown in Fig. 13. This last might be called a five- 

 celled stage with the beginnings of other divisions which soon become 

 more prominent. From the vegetative pole of this stage one part 

 of the first segmentation plane is visible. What may be called an 

 eight-cell stage seems to be rather irregularly derived from a four- 

 cell stage by the gradual addition of furrows which in any case can- 

 not be recognized as parts of an equatorial cleavage plane. Several 

 blastomeres may come to be marked off in this way, their limits 

 become well defined at the animal pole, but in all eggs examined by 

 me they were not well defined a short distance away from it. 



The original furrows of the first and second planes, although a 

 little more pronounced than the others, can hardly be told from them, 

 their course is so changed by a shifting of the parts ; however, they 

 are usually of greater extent and are deeper. 



Quite a typical eight-cell stage is shown in Fig. 16. Fig. 17 

 shows the same egg from below, the first segmentation plane com- 

 pletely encircles the egg. Fig. 18 shows another similar eight-cell 

 stage from the side. 



Sections of an eight-cell stage show quite a large segmentation 

 cavity under the few well defined blastomeres of the animal pole, 

 these few central cells in the region of small yolk granules were 

 well marked off from the rest of the egg while other divisions seem 

 to have only penetrated a short distance into the mass of yolk. Even 

 in the case of the first segmentation plane which nearly encircles the 

 egg, the central mass of yolk still remained undivided (Fig. 50). 



Wilder recognized a horizontal cleavage plane about the upper 

 pole of an eight-cell stage, but in the eggs examined by me the eight- 

 cell stage was more irregular and I could not recognize such a plane 

 of cleavage very clearly. ISTow and then I obtained stages which 

 were quite regular and approximately sixteen-cell stages such as Fig. 

 19, but I could trace no single regular cleavages which brought about 



