330 ttlCHAED ASSHETON. 



Keibel's (18, 19) work, which treats of the pig (Sus scrofa 

 domesticus), contains no reference to earlier embryos than 

 those of fourteen days after fertilisation. At fourteen days 

 the embryonal area is well defined and oval; the primitive 

 streak is distinct, and the whole blastodermic vesicle has 

 grown to an extraordinary extent, varying from half to a 

 whole metre in length. 



A younger stage of the pig's development has been described 

 by Weysse (25), who obtained some thirty specimens of about 

 the ninth to eleventh days, which he states present "pheno- 

 mena unusual in the ontogeny of the Mammalia." 



My specimens, which I propose to describe at the present 

 time, are of stages prior to any described by the above authors, 

 and date from the second day after fertilisation to the tenth. 



They include the earliest phases of segmentation, the forma- 

 tion of the blastodermic vesicle, and the definite establishment 

 of the epiblast and hypoblast layers of the future embryo. 



Method of treating the Specimens. 



The great difficulty of finding the embryos has made it 

 almost impossible to examine many in the fresh state. The 

 size of the cavity of the uterus of a pig is so great, and the 

 foldings of the mucous membrane are so complicated, that 

 a search to find them in situ proved most unprofitable. 

 Accordingly, with very few exceptions, the whole of the 

 material upon which I have worked was preserved in the way 

 I am about to describe before any examination of it was made. 



The ova, which, while in the Fallopian tube, can of course 

 be very easily found, unfortunately in the pig travel very 

 rapidly down the Fallopian tube, and pass into the uterus 

 during about the third day, before segmentation has proceeded 

 further than the four-segment stage. 



The ova are extremely small, and receive no additional 

 investment from the walls of the oviduct, and are quite 

 invisible as they lie among the folds of the uterus wall. J3y 

 scraping the mucous membrane of the uterus a specimen may 

 occasionally be procured, but by far the most satisfactory 



