332 RIOHyVRD ASSHETON. 



List of embryos obtained arranged according to age 



List of embryos arranged according to stage of develop- 

 ment: 



A. Prior to the 5 -segment stage 13 



B. The 5 -segment stage to morula, with surface cells approxi- 



mately equal 12 



C. Commencement of blastodermic vesicle to oldest retaining 



zona 33 



D. From rupture of zona to rupture of Rauber layer . . 20 



E. Formation of embryonal area to first sign of primitive streak 22 



Embryos of Stages A and B. 



The dates which are given must be regarded as approximate 

 only. 



In the pig it would seem that fertilisation may occur about 

 the end of the first day, or may be postponed two or three 

 days after union. 



I have made no observations upon the process of fertilisa- 

 tion, and merely draw the above conclusions from finding 

 specimens in identical conditions in pigs killed upon the 

 fourth, fifth, or sixth day. It is accordingly very difficult to 

 obtain any given stage. 



Embryos frequently difi'er very greatly in the same uterus. 

 For instance, in one case I had ova in two segments, nine 

 segments, and completed raorulse from the same horn of the 

 uterus. 



The youngest stage that I have is a two-segment stage 

 (PI. 25, figs. 1 and 2). This was taken from the uterus of a 

 pig on the sixth day. It was preserved in Flemming's solution 

 (weak formula). The embryo is protected by a thin zona 



