EMBRYO OF TWENTY-EIGHT DAYS. 



145 



degree of curvature of the back, in consequence of which specimens differ in 

 general form and in maximum length, though agreeing closely in structure. 



Embryos of the Second, Third, and Fourth Months. 



The following descriptions and figures illustrate the external form, no at- 

 tempt being made to describe the anatomical structures. 



Twenty-eight to Thirty Days. — Embryos of 8 to 10 mm. Figs. 83 and 84 show 

 an embryo of 9.0 mm. There is a general resemblance to the stage last described, 

 but the following changes may be especially noted, the first and most conspicu- 

 ous being the modification of the 

 limb-buds, which have increased 

 greatly. The end of each bud has 

 become somewhat flattened, rounded 

 in outline, and marked off by a slight 

 constriction from the portion of the 

 limb-bud near the body. The ex- 

 panded, flattened distal portion of 

 the limb is the anlage of the hand 

 in the upper limb, of the foot in the 

 lower. The position of the limbs is 

 also characteristic, the arm project- 

 ing in a line nearly parallel with 

 the axis of the body, while the leg 

 stands out at a wide angle. The 

 cervical sinus has entirely disap- 

 peared externally by closure of its 

 orifice. The nasal pit has con- 

 tracted and deepened so as to have 

 lost its original saucer-like form and 

 more truly to deserve its name of 

 pit. The large cavity of the fourth ventricle of the brain has become more 

 conspicuous. The umbilical cord has increased in length. From its distal end 

 springs the amnion, beyond which there passes out from the cord the narrow 

 stalk of the pear-shaped yolk-sac. The cavity in the interior of the umbilical 

 cord is a prolongation of the ccelom of the embryo, and through this coelom the 

 yolk-stalk takes its course. It bears blood-vessels which ramifv over the 

 surface of the yolk-sac proper. Unfortunately in the figure the amnion is 

 not shown. 



Thirty-one to Thirty-two Days. — Embryos of 10 to 12 mm. As a typical 



Fig. 83. — Human Ovum with Embryo of 9.S mm. 

 The Chorion Has Been Partly Removed to 

 Show the Embryo. X 2 diams. — {Minot Collec- 

 tion, 27 j.) 



