A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 419 



in a distinctly younger stage of development than either of these, must be con- 

 sidered as correspondingly younger, though it can not be younger than 11 days. 



The morphological description of the Reichert (1873) specimen is in many 

 respects inadequate. From its dimensions, however, it must fall in our second 

 group, the embryos of which have a primitive groove but no head-process canal. 

 The patient in this case died of suicidal poisoning 14 days subsequent to the omis- 

 sion of her menstrual period. If an additional day is allowed for the interruption 

 of her period it makes a minimum age of 15 days for this group. There are no 

 clinical data from the other specimens of this group which would make possible a 

 conclusion as to the maximum age for this stage of development. It must he some- 

 where between the maximum age of the preceding and that of the succeeding 

 group that is, between 15 and 19 days. 



In the third group data as to maximum and minimum age are furnished by the 

 Frassi (1907) and Eternod (1898) ova. In the Frassi case the patient reported that 

 her last menstrual period began two weeks before the hysterectomy, but upon 

 subsequent careful questioning it was learned that she had missed her period 14 

 days prior to the operation. Allowing an additional day for the interruption of the 

 period, this leaves a minimum age of 15 days. In the Eternod specimen the abor- 

 tion of a well-preserved, normal ovum followed 21 days after a single coitus. Assum- 

 ing one day for the process of fertilization, there are left 19 days as the maximum 

 age. From the foregoing it is apparent that the period of development from the 

 time of the formation of the ectodermic vesicle (Bry ce and Teacher) up to the time j ust 

 preceding the appearance of somites (Eternod) does not cover more than 8 days 

 (eleventh to nineteenth day). 



While the data in each of these selected cases tend to substantiate that of the 

 others, some of them nevertheless are not entirely above question. In the case of the 

 Bryce-Teacher specimen one might raise the point that, although at least 1 1 days 

 elapsed from fertilization to the beginning of the abortion, we can not be sure that 

 the embryo continued in its development up to that time. It may have died a few 

 days previous, thus making a minimum age of less than 11 days. In the Fetzer 

 case we are allowing 24 hours for fertilization following the last coitus, which is 

 probably adequate, although we know that spermatozoa may retain their vitality 

 much longer than this. In the Merttens specimen there can be no doubt as to the 

 accuracy of 15 days as the maximum age. The same is true of the Reichert and 

 Frassi specimens, in which 15 days is certainly their minimum age. In the case of 

 the Eternod specimen we must depend on the reliability of the patient's statements 

 and at the same time assume that 24 hours is sufficient for fertilization. That we 

 are safe, however, in accepting the confirmatory trend of these data is supported 

 by the recent observations of Huber on the rat, in which it was possible to obtain 

 timed specimens. 



In addition to the above 6 specimens in which decisive data as to maximum and 

 minimum age are present, there are 1 1 other specimens of the presomite period, includ- 

 ing our own, which have available clinical records. These are arranged in the order of 

 their development in chart 3, and the probable duration of pregnancy in the individ- 



