THE ORGANS OF THE INNER GERM-LAYER. 301 



descends to the left pelvic region, where it is continued into the 

 sigmoid flexure and rectum. Therefore there are distinguishable in 

 the colon, even in the third month, the ccecum, the transverse and 

 the descending colon. An ascending colon is still wanting. It is 

 formed in the succeeding months (fig. 165 B) by the gradual sinking 

 down of the coecum, which was at first under the liver, until in the 

 seventh month it is below the right kidney, and from the eighth 

 month onward descends past the crest of the ilium. 



Meanwhile the ccecum has increased in length and toward the 

 end of pregnancy is a rather large appendage at the place of tran- 

 sition from the small to the large intestine. It early exhibits a 

 want of uniformity in development (fig. 165 B bid). The terminal 

 part, which often embraces more than half its length, does not keep 

 pace in its growth with the more rapidly enlarging proximal portion ; 

 the former is designated as the appendix vermiformis, the latter as the 

 coecum. At the time of birth the vermiform appendage is still not 

 so sharply differentiated from the coecum as it is a few years later, 

 when it has been converted into an appendage of the size of a goose - 

 quill and 6 to 8 cm. long. 



Within the region embraced by the bends of the large intestine, 

 the small intestine, which is derived from the descending arm of 

 the loop, is disposed in more and more numerous folds owing to 

 its extensive growth in length (fig. 165 B). 



At first all regions of the intestine from the stomach onward are 

 so united to the lumbar region of the vertebral column by means of 

 a common mesentery (mesenterium commune) that they can move 

 freely (fig. 165 A and ). The mesentery is naturally influenced by 

 the increase in the length of the intestine, inasmuch as its line of 

 insertion on the intestine exceeds in length many times the line of 

 origin at the vertebral column (radix mesenterii), and is thereby laid 

 into folds like a frill. Such an arrangement of the mesentery is 

 found to be the permanent condition in many Mammals, as in the 

 Dog, the Cat, etc. 



But in the case of Man, from the fourth month onward, the 

 arrangement of the mesentery is much more complicated. There 

 occur changes which may be briefly characterised as processes of 

 fusion and concrescence of certain portions of the mesenterial lamella 

 with contiguous parts of the peritoneum, either of the posterior wall 

 of the body-cavity, or of neighboring organs. They affect the 

 mesentery of the duodenum and colon, which is always present in 

 the first half of embryonic development. 



