134 GEORGE W. CORNER 



would seem that fertilization may occur about the end of the first 

 day or may be postponed until two or three days after copulation — 

 a conclusion which he draws from finding embryos of the same 

 stage in two sows killed on the fourth, fifth, and sixth day post 

 coitum. Likewise, embryos in the same uterus may vary rather 

 markedly as to their state of development, for instance, one uterus 

 contained ova of two segments, of nine segments, and completed 

 morulae. For this reason it is possible to give only an approx- 

 imate time schedule of early development. The ova pass down 

 the tube rapidly and enter the uterus about the fourth day post 

 coitum. Assheton did not find any stage further advanced than 

 four blastomeres in the Fallopian tubes. (A specimen found by 

 the present writer and Mr. Felix H. Hurni contained ova of two, 

 four, and six blastomeres, all in the tube.) Assheton found that 

 various sows killed on the sixth day presented uterine embyros 

 from the stage of six blastomeres to fairly well-developed blasto- 

 dermic vesicles. By the seventh day the zona pellucida has 

 usually disappeared and the inner cell mass of the early vesicle 

 has differentiated into two layers, the epiblast and the hypoblast. 

 By the twelfth day the great elongation of the blastodermic 

 vesicle which is so characteristic of the pig, is well under way and 

 the vesicle is already 10 to 12 mm. long. By the fourteenth day 

 each vesicle may measure 20 cm. ; in the embryonic area the prim- 

 itive streak is well developed and there are from one to three so- 

 mites. In addition to Assheton's studies, thirty embryos of the 

 ninth, tenth and eleventh days have been described by Weysse 

 ('94), and from the fourteenth day to about the twenty-fifth we 

 have the accurate tables of Keibel ('97). For older (foetal) 

 stages, no good age-length ratios have been determined. The 

 period of gestation is usually 116 to 120 days. It is stated that 

 sows undergo oestrus and may become pregnant again five weeks 

 after littering. 



During the progress of this investigation the ovaries and uteri 

 of several thousand sows have been examined macroscopically, 

 and the corpora lutea of about 300 have been studied under the 

 microscope. The permanent preparations upon which the fol- 

 lowing description is based comprise sections from the Graafian 



