HUMAN DEVELOPMENT 



273 



r,. 



AORTA 

 MESONEPHROS 



LIVER 

 UMBILICAL VEIN 



Fig. 182. The mesoneph- 

 ric tubules, stomach, and liver 

 in a pig embryo. 



>^._ TAIL 



Fig. 183. A cross section 

 of the mesonephric tubules of 

 a pig embryo. 



geal regions of the chick and the pig. Figure 181 is a photograph of a section 

 through the pharynx of the pig embryo. Note the deep branchial grooves and 

 the endodermal pouches. 



In the development of the excretory system of the pig embryo the same 

 succession of pronephros, mesonephros, and metanephros is found as was 

 previously described for the chick embryo. Figure 182 shows a dissection of 

 the large mesonephros, and Figure 183 shows a cross section through this 

 organ. 



These examples should serve to show that the development of the mam- 

 malian embryo follows quite closely the pattern found in the chick embryo. 

 Furthermore, a comparison of a section of the human embryo with that of 

 the pig shows an almost identical structure (Fig. 176). It is therefore clear 

 that there is nothing unique about the early development of the human em- 

 bryo. The features which differ from those of the chick are found in the 

 manner of development of the extraembryonic membranes and the placenta. 



Human development 



Although data on very early human development are few, the picture of 

 development reconstructed from embryos in various stages of development 

 gives us a fairly clear idea as to the morphological changes in human develop- 

 ment. To be sure, there are large gaps, since the earliest human embryo which 

 has been described at the time of this writing is about seven days old. How- 

 ever, the pattern indicated is so clear that we can reconstruct the missing 

 stages from other animals. 



