CYCLIC CHANGES IN THE OVARIES AND UTERUS OF THE SOW, ETC. 



Degeneration of ova in non-preqnant animals. 



127 



No ova were found in animals of later date. 



We shall not be far wrong, therefore, in assuming that the unfertilized ovum of 

 the sow disappears by degeneration in utero at about the seventh or eighth day after 

 its discharge from the follicle. Degeneration of the unfertilized ovum is charac- 

 terized by division of the cytoplasm into rounded masses, which are usually not 

 uniform in size, but sometimes are sufficiently regular to simulate segmentation. 

 These masses ultimately contract so as to present a shrunken appearance within a 

 zona pellucida which is often swollen but at the same time less refractive. The 

 zona pellucida frequently loses its spherical shape and at the last may present only 

 a vague, flattened halo, containing minute punctate granulations, around a few 

 shrunken masses of cytoplasm. 



Of still more importance, in affording a basis for correlating the anatomical 

 events of the reproductive cycle, is the question of the rate of growth and the time 

 of implantation of the fertilized ovum. Fortunately for our purposes, pig embryos 

 of known ages have now been seen at all stages of early development. The present 

 writer, with Amsbaugh.(1917), had the opportunity of completing the series by 

 the observation of ova immediately after fertilization, before conjugation of the 

 pronuclei. The careful studies of Assheton (1898) on about 100 embryos carry the 

 history from the stage of two blastomeres until the tenth day, and 30 more speci- 

 mens of the ninth, tenth, and eleventh days have been described by Weysse (1894). 

 From the fourteenth to about the twenty-fifth day we have the detailed data of 

 ^KeibePs Normentafeln (1897). The following account of the early embryology 

 of the pig is based upon the contributions just mentioned and upon personal study 

 of a series of specimens of all stages. 



Segmentation begins in the tube and continues after passage of the ova into 

 the uterus; no specimen beyond the stage of 6 blastomeres has yet been observed 

 in the tube. Assfeeton found that various sows killed on the sixth day contained 

 uterine embryos from 6 blastomeres to fairly well-developed blastodermic vesicles. 

 By the seventh day the zona pellucida has disappeared and two layers, epiblast and 

 hypoblast, may be distinguished in the inner cell-mass of the vesicles. By the 

 eighth or ninth day the vesicles begin to be wrinkled and then to undergo the 

 remarkable elongation which is so characteristic of early pig embryos. By the 



