DEVELOPMENT OF THE PIG DURING FIRST TEN DAYS. 333 



radiata, which is not thickened by the addition of any mucous 

 or albuminous layer. 



The slight and very thin irregular external coat to the zona 

 radiata is seea upon higher magnification to be caused by 

 numerous spermatozoa, which seem to have adhered to the 

 zona radiata. 



The embryo lies freely within the zona radiata. Both 

 segments appear plainly divisible into two parts, an outer 

 clearer layer which extends round the greater part of an inner 

 much darker part, within which lies the nucleus. 



The outer layer, which shows a very fine reticulation or 

 granulation, is only very slightly darkened by the fixing solu- 

 tion. On the other hand, the inner portion is very much 

 blackened, owing to the action of the osmic acid upon the 

 numerous oil globules whose presence characterises this part 

 of the segment. 



In a fresh specimen these globules are rather dark and of an 

 olive-green colour. Ordinary stains have no effect upon them. 

 Osmic acid is the only reagent that I have found to colour 

 them. In this respect, among others, they differ very much 

 from the yolk granules of reptilian, avian, and elasmobranch 

 eggs. 



The nucleus, like the outer layer, has none of these oil 

 globules, and so the centre of each segment appears more 

 transparent. The nucleus of each of the segments of this 

 specimen is distinctly visible, and is spherical. 



Two polar bodies are present. 



Fig. 2, which is a side view, shows very distinctly the rela- 

 tive extent and position of the outer clearer zone and the 

 inner oil-bearing mass. 



Fig. 3 was drawn from a fresh specimen taken from the 

 upper end of the uterus within 10 mm. of the opening of the 

 Fallopian tube. It was found with three other specimens, two 

 of which were in the four-segment stage, the other in five 

 segments. 



The nuclei could not be distinctly seen, but their presence was 

 indicated by the slightly greater transparency of the central 



