152 



THE HUMAN EMBRYO. 



illustrate the variations in the proportions of embryos and to show a slightly more 

 advanced stage, figure 104, of a 17.8 mm. embryo is given and also figure 105, 

 A, B, giving two views of an embryo of 18.1 mm. All three specimens are proba- 

 bly normal, for it is known that variation is much greater during development 

 than in the adult, a fact which is to be explained in large part by the temporary 

 accelerations or retardations of the development of single organs or regions, which 

 are subsequently compensated for. 



Embryos of Forty Days, 19 mm. The head has risen far toward its definite 

 position, with the result of a very rapid apparent increase in the total length of 



FIG. 102. HUMAN EMBRYO OF 14.5 MM. SERIES 1003. X 5 diams. 



the embryo. The change of position of the head results in bringing the mid-brain 

 finally directly above the hind-brain, 'the embryo being conceived as having the body 

 vertical. During the .elevation of the head the concavity (Nackengrube) at the back 

 of the neck is gradually obliterated. In both head and rump the external modeling, 

 which in earlier stages indicated more or less the position of the internal organs, 

 has become blurred, and in the next stage is found to have nearly or quite disap- 

 peared. The maxillary processes have met and united in the median line. The 

 anlages of the eyelids have developed. The concha of the ear is indicated. The 

 arm reaches beyond the heart; the fingers appear as separate outgrowths. 



Embryos of Fifty Days, 21 mm. The author has a fair specimen which came 

 into his possession with no history whatever, but it agrees very closely with His's 



