EMBRYO OF THIRTY-EIGHT DAYS. 



149 



cavity is furnished in which there are always found, as indicated in figure 84, 

 several coils of intestine. This protrusion of a portion of the intestinal canal, 

 and sometimes even of a small portion of the liver, into the extra-embryonic 

 ccelom of the umbilical cord is a constant phenomenon. It begins at a somewhat 

 earlier stage and continues for a considerable period. This curious condition has 

 been observed in many different kinds of mammals in the corresponding stage. 

 Later on, the viscera are entirely withdrawn from the umbilical cord and the 

 cavity itself is wholly obliterated. Figure 87 is inserted to illustrate the 

 relation of the foetal appendages to the embryo at this stage and in slightly 

 younger embryos. The figure shows clearly 

 that the umbilical cord is a hollow prolon- 

 gation of the body-wall or somatopleure 

 of the embryo, and that the amnion springs 

 from its distal end. The yolk-stalk, V.S, is 

 very long and narrow. Its entodermal 

 cavity is obliterated. It is the representa- 

 tive of the original broad connection be- 

 tween the yolk-sac and the entodermal 

 cavity of the embryo, although it is now 

 only a small appendage of a loop of the 

 intestine. It bears the blood-vessels which 

 run from the embryo and ramify upon the 

 yolk-sac. On the caudal side of the umbili- 

 cal cord we find the tissue of the original 

 body-stalk in which runs the allantoic vein, 

 Al. i>, and the two allantoic arteries, Al. a, 

 which ramify upon the chorion, Ch. 



Thirty-eight Days. Embryo of 16 mm. 

 in a chorionic vesicle of 45 by 40 mm. The 

 age of this specimen (Fig. 88) is known by 



estimate only. This stage represents the transition of the embryo to the 

 foetus, because after the fortieth day the form is distinctly human. The 

 head has risen considerably, and the back has straightened still more, the 

 rectangular neck bend thus becoming emphasized. The body has become 

 still more protuberant on the ventral side, and in side views the arms no 

 longer reach to the outline of the body. 



Forty Days. Embryos of 19 mm. The head has risen far toward its de- 

 finite position, with the result of a very rapid apparent increase in the length of 

 the embryo. The change of position of the head results in bringing the 



FIG 



HUMAN EMBRYO OF ABOUT 16 MM. 

 X 5 diams. (After W. His.) 



