460 MALL. ' [Vol. V. 



Age. 



The nape-breech length measured 7 mm. and the vertex- 

 breech 6 mm. The study of mammalian embryos, as well as 

 a series of human embryos, tells us that this embryo cannot be 

 over a month old. From the results of post-mortem examina- 

 tions of women shortly before the beginning of menstruation, 

 Bischoff, Williams, Dalton, Leopold, and others place ovulation 

 two or three days before the beginning of menstruation. 1 Espe- 

 cially on account of the study of several cases in which the 

 earliest possible cohabitation took place a week or two after the 

 last menstrual period, embryologists and gynecologists reckon 

 the duration of pregnancy from the beginning of the first period 

 which has fallen out. 



So in order to estimate more accurately the age of this em- 

 bryo, we must subtract twenty-eight from the time which has 

 elapsed since the beginning of the last period (fifty-two days), 

 and add two for the time between ovulation and menstruation. 

 The shape and size of this embryo correspond with that described 

 by others as the fourth week, and twenty-six days is in all proba- 

 bility its age. 



External Form. 



The embryo is flexed upon itself, forming almost a circle 

 (PI. XXIX., Fig. 1). The head shows the outline of the brain 

 within, and also a marked elevation over the region of the Gas- 

 serian ganglion. The nasal pit is a large shallow depression, 

 being well exposed on both sides. The lense is small, and 

 is surrounded by a groove which is continued between the supe- 

 rior maxillary process and nasal pit. 



Three branchial arches are visible on the right side, and four 

 on the left. The ventral end of the first is bulbous, while from 

 its dorsal end the superior maxillary process arises. The 

 second is also bulbous on its ventral end, the major portion of 

 the trunk hanging over the third arch. This is the embryonic 

 operculum which will finally close the sinus precervical is. The 

 third arch lies more towards the median line, that is, it is 

 within the sinus praecervicalis. The fourth arch is visible only 



1 His, Anat. tnensch. Embryonen, II, Leipzig, 1882. 



