DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 157 



The length of the embryo is considered as the length of a straight line drawn 

 from the apex of the cervical flexure to the apex of the sacral flexure (neck- 

 rump length, Nackensteisslange; see Fig. 124). During the second month and 

 later, or in embryos of more than 20 mm., the body becomes more nearly 

 straight and the measurement is taken along a straight line from the apex of the 

 cephalic flexure to the apex of the sacral flexure (crown-rump length, Scheitel- 

 steisslange; see Fig. 126). 



Owing to the changes in curvature of embryos during development, no 

 one system of measurement will give uniform results for all stages. In this 

 country it is the general practice to measure the greatest length of the embryo, 

 in its natural attitude, along a straight line. The measurement does not 

 of course include the extremities. At certain stages this length corresponds 

 with the neck-rump length, at other stages with the crown-rump length, at still 

 other stages with neither. 



Relation of Age to Length. — Not infrequently the history of an embryo 

 is not obtainable, and in such cases the age must be inferred from what is known 

 concerning the relation of the age to the length of the embryo. The age can be 

 computed approximately by this means, although there is a possibility of error. 

 Embryos of the same age are not necessarily of the same length, since conditions 

 of nutrition, etc., determine not only the size of the embryo but also the degree 

 of its development. In the later stages of development the limit of error is not 

 so important, but in the younger stages the difference of a day or two means 

 much. 



His estimated the ages of a number of embryos from available data as 

 follows : 



Embryos of 2-2J weeks measure 2.2-3 mm - (neck-rump length). 



Embryos of 2^-3 weeks measure 3-4.5 mm. (neck-rump length). 



Embryos of 3 J weeks measure 5-6 mm. (neck-rump length). 



Embryos of 4 weeks measure 7-8 mm. (neck-rump length) . 



Embryos of 4 J weeks measure 10-n mm. (neck-rump length). 



Embryos of 5 weeks measure 13 mm. (neck-rump length). 



More recent researches on the rate of development in the lower Mammals 

 tend to show that development proceeds relatively slowly during the earliest 

 stages, and then goes on with increasing rapidity for a time. In the rabbit, for 

 example, it has been shown that the embryonic disk is but slightly differentiated 

 at the seventh and eighth days, while at the tenth day the embryo possesses 

 branchial grooves and primitive segments. If this peculiarity in the rate of 

 development occurs in the human embryo, the ages assigned to the earlier 

 embryos by His must be increased. 



Mall's formulae for estimating age, deduced from observations on a large 

 number of embryos, are as follows: In embryos of 1-100 mm. the age in days 



