THE EARLY DEVELOPMENT OF THE MAMMAL 411 



The general changes in size and weight of the embryo and 

 foetus during the entire intra-uterine period are summarized in 

 the accompanying table. 



TABLE SHOWING THE AVERAGE WEIGHT AND LENGTH OF 

 THE HUMAN EMBRYO AND FCETUS. 



Compiled from Jackson (weight) and Mall (length). (The column headed 

 CH gives the length as measured in a straight line from the crown of the head 

 to the heel; that marked CR gives the "sitting height," or length from the 

 crown to rump or sacral flexure.) 



Before leaving the subject of the development of external 

 form we should add a few details regarding the development 

 of the facial characteristics and of the external genitalia. 



THE DEVELOPMENT OF THE FACE 



We may take as our starting point here, a stage of 2.6 mm. 

 (probably about thirty days) already described and figured 

 (Fig. 164). At this time the first gill cleft is unreduced, the 

 otocyst is not yet closed, and the optic vesicles are entirely lateral 

 in position. The fore-brain region hangs down over the deep 



* Age probably underestimated. 





