4 



same time, on comparing Figs. i. 2. 4. and 5 it is clearly 

 demonstrated how much more developed the lower part 

 of the abdominal and pelvic cavities is in the older than 

 in the younger stages. This pelvic growth is perhaps the 

 single most potent factor in the provision of room for the 

 intestines. 



In a recent paper * Mall finds this same state of affairs to 

 exist in the human embr\^o. 



The diaphragm is distinctly more concave in the older 

 than in the younger embryos. As development proceeds 

 this concavit>' of the diaphragm increases the capacity of the 

 abdominal cavity, which is easily understood on comparing 

 the Figs. I to 5. 



We have alread\- seen that the liver develops in the direc- 

 tion of least resistance, and so it is not surprising that it should 

 change its shape in order to conform to the concav^ity of the 

 diaphragm, and thus to develop cephalad, so leaving more 

 space caudad for the intestines. 



The vitelline vessels (omphalo-mesaraic, superior mesen- 

 teric) are certainly much larger, in proportion to the size of 

 the embryo, in the younger than in the older stages. I 

 am inclined to believe that they are actually very little 

 larger in an embr^^o of 38| millimetres than in one of 15 

 millimetres, but my sections have not allowed me to surely 

 determine this point. Be that as it may, these vessels in tlie 

 young embryo occupy a very appreciable amount of space, 

 and are an important factor in the consideration of the con- 

 tents of the abdominal cavit}'. They have one curious char- 

 acteristic, and that is that in embryos of from 8 to 15 

 millimetres the diameter of both the artery and the vein 

 gradually increases as they approach the yolk sack. 



This interesting arter}- will probably be the subject of a 

 future investigation, which I hope soon to complete. 



The Wolffian bodies are proportionately larger in the 

 younger than in the older stages, but on the otlier hand it 

 must be remembered that a portion of the space thus gained 

 in the older embr>'os is partially utilized in the development 

 of the true kidneys. 



' Anat. Anzeiger. Bd. XVI ., 1899. 



