DEVELOPMENT OP THE PIG DURING FIRST TEN DATS. 337 



very considerable difference in tlie minute structure of the 

 segments as compared with an earlier stage. 



Although the oily *^yolk" vesicles are still present, but in 

 diminished numbers, there is certainly a tendency to the 

 formation of other spaces or vacuoles. I cannot say whether 

 these spaces are the commencement of the cavity of the blasto- 

 dermic vesicle or not. 



Fig. 14 is a median section through a specimen such as 

 that represented by fig. 7. It was taken from the uterus at 

 six days. 



It is very difficult to detect in sections any boundaries 

 between the cells. It would not, however, be right to say 

 that the embryo is a vacuolated protoplasmic mass containing 

 nuclei, for surface-view specimens treated with silver nitrate 

 show the lines of division between the cells very clearly. Here 

 and there, also, there is in some of the sections a trace of a 

 cell boundary, more especially where it would mark off a peri- 

 pheral from an inner mass cell. 



There is also a distinctly radiate arrangement of the proto- 

 plasm round certain centres, by which the sections (v. fig. 14) 

 are marked out into cellular areas. The arrangement of the 

 nuclei round the periphery should be noted. 



This concludes all the evidence I have to offer with regard 

 to the process of segmentation of the ovum in the pig. 



The most noteworthy feature is the very great dissimilarity 

 of size of the segments after the four-segment stage. I un- 

 fortunately have only one certain specimen in the two-segment 

 stage. In this I can see no difference in size or nature 

 between the two. 



In the four-celled stage there is always a slight difference 

 in size, but at the eight- to twelve-segment stage the difference 

 is most marked, and recalls the great difference which we find 

 in many molluscan ova. In no case, however, have I found 

 any difference perceptible except in size. The smaller seg- 

 ments form a cap upon the larger. 



This great inequality is more marked than in any other 

 mammalian segmentation. Bischoff shows nothing like it in 



