DEVELOPMENT OF THE RECTUM 



39 



analis a^aiii lies at about the level of the third or fourth sacral 

 vertebra, while the anal opening lies at about the same level as 

 the tip of the coccyx. 



In embryos of 31 mm. (H. 57) and 35 mm. (H. 51) the bulbus 

 analis reaches only as high as the fifth sacral vertebra, while the 

 anal opening presents the same relations as in the preceding stage. 



In an embryo of 50 mm. (H. 115, fig. 6, B) the bulbus analis is 

 relatively much shorter. Its upper limit now lies on a level with 

 the second or third coccygeal vertebra, while the anal opening 

 is found on about the same level as the tip of the coccyx. At 



CX.Spll. 



^ B 



Fig. () Sagittal section through the anal regions of human embryos. A, 28.8 

 mm., (H.E.C. 1598) and B, 50 mm. (H. 115). X 6. all., allantois; anip.r., ampulla 

 recti; bul.a., bulbus analis; bul.l., bulbus terminalis; C.I to IV, coccygeal verte- 

 brae; ex.sph., external sphincter muscle; Icv.a., levator ani muscle; per.c, peri- 

 toneal cavity; S.I. to V., sacral vertebrae; sym.p., symphysis pubis; z.ni^., zona 

 intermedia. 



140 mm. the curvature of the sacrum and coccyx is less than 

 before, and the bulbus analis lies entirely below the tip of the 

 coccyx. At birth practically the adult relations are met with 



(fig. 24). 



The limit hetween the entoderm and ectoderm 



Retterer ('90) in a study of the early stages of embryos of the 

 sheep, pig, and rabbit, concluded that the mucous membrane 

 of the anal region is derived directly from the ectoderm and the 

 underlying connective tissue. Herrmann ('80) likewise believed 

 that the whole of the pars analis recti is derived from the ecto- 



