284 PROCEEDINGS OF THE AMERICAN ACADEMY. 



to the difficulty of securing sufficient material in successive stages of 

 growth, and there is the further difficulty that it is impossible to deter- 

 mine iu the great majoiity of cases the exact age of the embryos, since 

 spermatozoa may remain for several hours, days, or even months, within 

 the body of the female before fertilization takes place. And further, 

 it has long been known that, especially during the earlier phases of de- 

 velopment, ova which were apparently fertilized at the same time grow 

 at very different rates, so that within one uterus we may find embryos 

 illustrating several stages of ontogeny. For these reasons I cannot 

 give exact ages for the embryos 1 am about to describe, though in 

 almost every case I can give the period which has elapsed between 

 coitus and the time when the sows were killed and the embryos 

 obtained. 



2. Material. 



The present paper is based on the results obtained by the study of 

 thirty embryos taken from four sows. In all I had nine sows served 

 at different times, and these were killed at from nine to eleven days 

 after coitus. Only three of the nine proved to be pregnant at the 

 time they were killed, and of these three, two were killed ten days 

 and the other eleven days after copulation. P^ach of the two sows 

 killed on the tenth day contained eleven embryos in varying stages of 

 development, though all the embryos in one were much less advanced 

 than those in the other, as I conclude from the difference in the size 

 both of the embryos and of the cells of which they are composed, and 

 from the difference in the size and structure of the germinal disks. 

 The third sow, killed on the eleventh day, contained only four em- 

 bryos, two of which were as old as the oldest of those from the first 

 two sows ; the other two were younger. The remaining four embryos 

 of the thirty were found in a sow which had been served before coming 

 under my control, and were apparently in about the same stage onto- 

 genetically as some of the older embryos in the first two cases men- 

 tioned above. The uterus in which these last embryos lay, contained 

 six in all, but two were so badly folded as to be unavailable as far as 

 the study of the germinal disk was concerned. These embryos were 

 obtained by opening a large number of apparently empty uteri ; in this 

 way 1 have found about 3% of the individuals examined pregnant, but 

 all of the embryos with the exception of the six just mentioned were 

 in a much more advanced stage of devolopment, so that they afford no 

 information on the subject matter of the present article, and I shall 

 reserve the consideration of them for a future paper. 



