SEGMENTATION OF THE OVUM OF THE SHEEP. 211 



Fig. 7 represents a section passing through the centre of 

 this segment. This separation of one segment differing from 

 the other seven is very remarkable, because there is no trace 

 of such a difference in the former specimen (fig. 4). 



Unfortunately I had preserved the six-segment specimen in 

 Flemming's solution, and so a comparison between the two 

 is not so perfect as might be. 



I have another specimen in the eight-segment stage taken 

 from another sheep. It was found in the upper end of one 

 horn of the uterus of a sheep killed on the fourth day. It was 

 in chromic acid for forty-eight hours. 



This specimen was stained with borax carmine and mounted in 

 two groups of four cells in Canada balsam, as shown in fig. 34. 



The variation in size of the several segments may be recog- 

 nised in the drawing. I could see no difference at all as 

 regards colour. In one segment, which is slightly larger than 

 any of the others, there is a group of spherical bodies which 

 are stained a little more deeply than the protoplasm in which 

 they are embedded. 



It is a noteworthy fact that in two specimens of the eight- 

 celled stage of the embryo one of the segments differs from all 

 the others. On the other hand, I have two specimens in the 

 eight-cell stage taken from the same horn of a uterus, neither 

 of which shows any sign of difference between the several seg- 

 ments. Both, however, were stained differently from the before- 

 mentioned specimens. 



Fig. 5 represents the middle section of a series cut through 

 one of them. The cells are arranged so as to leave a distinct 

 cavity, which may be called a segmentation cavity. This cavity, 

 which would seem to persist for some time, always contains 

 spherical and otherwise shaped masses of a substance in every 

 way similar to the cytoplasm of the segments. 



I have specimens of 15, 15, 16, and 17 segments respec- 

 tively, all of which show features similar to the eight-celled 

 specimen shown in fig. 5. They may be described as '' blas- 

 tulse," with the segmentation cavity filled with fragments of 

 cells. 



