THE EARLY DEVELOPMENT OF THE MAMMAL 407 



single stalk from the embryo now carries both the yolk stalk 

 and the allantoic stalk. This is known as the umbilical stall: 

 or cord, and its attachment to the embryo is the umbilicus. 

 Ultimately the entire yolk stalk becomes enclosed in the um- 

 bilical cord, and the yolk-sac itself is surrounded by the tis- 

 sues of the placenta as described below. 



Up to this time the correlation between age and size of the 

 embryo is very uncertain. According to the careful studies of 

 Mall the ages of most of the early embryos have been under- 

 estimated, and it is very probable that the events thus far 

 described have occupied about the first month of development. 

 From this time on, however, the age of the human embryo is 

 more certainly determined. 



During the sixth week of development the embryo measures 

 9.0-10.0 mm. in a straight line drawn from the apex of the 

 mid-brain to the sacral flexure or rump ("crown-rump" 

 length) (Fig. 167). The head, still the largest part of the 

 embryo, is beginning to be elevated on account of the straight- 

 ening out of the cervical flexure, and the whole body shows 

 considerably less curvature than before. A lense has been 

 formed opposite the small optic vesicle, and a pair of well- 

 marked olfactory pits has appeared on the under side of the 

 head, in front of the maxillary processes. Both maxillary and 

 mandibular processes are more prominent, while the posterior 

 visceral arches and clefts have become sunk in a depression, 

 the margins of which have nearly closed together forming a 

 cavity below the surface know r n as the cervical sinus. The 

 most anterior gill cleft (hyomandibular) is not included in this 

 cervical sinus, but in part remains on the surface of the neck, as 

 the rudiment of the external auditory meatus. The cervical sinus 

 later disappears entirely, along with the posterior gill clefts. 



The ventral body region is still protuberant, and anteriorly 

 three elevations can often be observed, marking the under- 

 lying auricle, ventricle, and liver. The limbs are somewhat 

 elongated, and the fore-limbs, which are always in a more 

 advanced stage than the hind-limbs, show some indications of 

 differentiation of the hand. The umbilical cord is elongating 



