THE EARLY DEVELOPMENT OF THE MAMMAL 403 



which is now very large and clearly differentiated into regions 

 by the development of flexures. 



The length of the embryo now affords a very unsatisfactory 

 index of the age or degree of development on account of the 

 considerable variability, and because of the bendings which 

 appear in the longitudinal axis. Apparently, shortly after 

 this time, the body becomes sharply bent downward into a 

 U-form just opposite the umbilicus, producing what has been 

 called the dorsal flexure. That this is entirely normal is, 

 however, still open to question. 



-Am 



Yks 



FIG. 163. Human embryo of thirteen or fourteen days. From Minot 

 (Laboratory Text-book of Embryology), after Kollmann. Al, Body stalk; 

 Am, amnion; Ht, heart; Md, medullary groove; ST, seventh somite; Yks, yolk- 

 sac. 



Figure 164 illustrates an embryo, enclosed in the amnion 

 and with yolk-sac attached, whose length, in a straight line, is 

 2.6 mm. and whose age was originally estimated at eighteen 

 to twenty-one days, though very probably it was approximately 

 one month (His's embryo, M). The entire blastodermic vesicle 

 or chorionic vesicle ("ovum") still measures approximately 

 10 mm. in diameter. This embryo shows many important 

 advances. No trace of the dorsal flexure remains, while both 

 the anterior and posterior extremities of the embryo are now 

 bent downward and inward. The body is also slightly twisted 

 so that the head lies toward the left, the tail toward the right 

 (the direction of this twisting is not fixed, for in other embryos 

 it may be in the opposite direction). The yolk-sac is some- 



