470 THE HUMAN EMBRYO. 



occurrence, but such evidence as we have is in favour of the 

 former. 



It is customary, however, to adopt His' original rule, and to 

 estimate the age of human embryos from the first day of the first 

 omitted period, and this method will be followed in this chapter. 

 It must be repeated, however, that this is done merely from 

 convenience, and from the absence of any other precisely formu- 

 lated system. Viewed on its own merits, His' rule will certainly 

 not apply generally. 



THE GENERAL HISTORY OF DEVELOPMENT OF 

 THE HUMAN EMBRYO. 



In this section the earliest stages in the formation of the 

 human embryo will be described, so far as they are at present 

 known ; and an account will be given of the external characters 

 of the embryo at the several stages up to the time of birth. 



These descriptions are in the great majority of cases taken 

 from Professor His' monograph, and, as explained in the pre- 

 ceding section, the ages given are those assigned by him to the 

 several stages. In the following sections the development of 

 the nervous, digestive, and other systems will be considered in 

 detail ; and in the concluding section the placenta, the foetal 

 membranes, and the relations of the embryo to the uterus will 

 be described. 



The actual length of an embryo is not always easy to deter- 

 mine, owing to the varying amount of flexure of the head and 

 body at different stages. By the length of an embryo, in the 

 following descriptions, is always meant the longest straight line 

 that can be drawn through it in the sagittal plane. In the 

 earliest stages of development this coincides fairly well with the 

 longitudinal axis of the embryo (Figs. 176 to 179); from the 

 beginning of the fourth week to the end of the fifth week (Figs. 

 200, 203, and 205) it is a line drawn from the prominent hump 

 at the junction of the head and body, to the pelvic region ; and 

 from the end of the fifth week onwards, as the head is gradually 

 lifted up by straightening of the neck (Figs. 211, 212), the 

 line once more approximates to the longitudinal axis of the 

 foetus. 



With regard to the general course of development, the first 



