48 HUMAN EMBEYOLOGY. 



As development proceeds the mid-gut and the cephalic (anterior) part of the 

 hind-gut form a U-shaped tube which possesses a cranial (anterior) and a caudal 

 (posterior) limb, and a ventral extremity which is connected with the yolk-sac 

 by a narrowed and elongated canal, the vitello-intestinal duct (Fig. 5*7). 



Upon the caudal limb of the loop, about the middle of its dorso-ventral height, 

 an enlargement appears which is the rudiment of the csecum and vermiform process 

 of the adult. After this rudiment has formed the caudal limb of the loop under- 

 goes rotation, being carried first to the left, then cranially, and finally to the right. 

 As it is carried to the right it crosses the cranial (later ventral) aspect of the 

 cranial limb of the loop, and when the rotation is completed the regions of the 

 jejunum and ileum, the csecum, the ascending and the transverse colon are 

 defined. 



After the rotation has occurred the tubular intestine formed from the mid-gut 

 and the anterior part of the hind-gut, undergoes rapid elongation and is thrown 

 into a number of coils. 



When the embryo has attained the length of 10 mm., and is a little over a 

 month old, the greater portion of the coiled gut passes through the umbilical 

 orifice into an expansion of the coelom formed in the proximal part of the umbilical 

 cord (see p. 47) (Fig. 63), which has replaced the allantoic or body-stalk as the 

 medium by which the embryo is attached to the chorion. The herniated coils 

 remain in the root of the umbilical cord until the embryo is about 40 mm. long, 

 and about ten weeks old, when they return to the abdomen, and the coelomic 

 space in the umbilical cord disappears. 



The Derivatives of the Posterior Part of the Hind-Gut. When the caudal 

 portion of the hind-gut is first enclosed its terminal extremity and its ventral 

 wall are bounded by the caudal portion of the primitive streak, which is bent 

 ventrally during the folding -off of the embryo. 



The terminal part of this portion of the gut becomes expanded, forming 

 a chamber called the entodermal cloaca, into the ventral parts of which the 

 ducts of the primitive kidneys, the pronephric or Wolfl&an ducts, open, one on each 

 side. 



The ventral part of the cephalic end of the cloaca is continuous with the 

 allantoic diverticulum, and the dorsal part with a tubular portion of gut which 

 forms the descending and possibly also the iliac and pelvic portions of the colon. 



As the temporary tail is formed and projected first caudally and then ventrally, 

 by the growth energy of the nodal point situated at the caudal end of the neural 

 tube, a diverticulum of the caudal end of the dorsal part of the cloaca is prolonged 

 into it, forming the tail gut. This soon becomes shut off from the cloaca. It 

 entirely disappears before the temporary tail is absorbed into the caudal end of 

 the body (Figs. 57, 62, 63). 



At a later period the cloaca itself is separated into a dorsal part, the rectum, 

 and a ventral part, the urino-genital chamber, by the formation of a septum, which 

 commences in the angle between the allantoic diverticulum and the ventral wall 

 of the cloaca, and is prolonged caudally till it reaches and fuses with the internal 

 surface of the cloacal membrane, which thus becomes separated into urino-genital 

 and anal portions, both of which disappear about the eighth week. 



In both sexes the urino-genital section of the cloaca is separable into three 

 parts : (1) a cranial part, which is converted into the urachus or middle umbilical 

 ligament ; (2) an intermediate part, which becomes the urinary bladder ; and 

 (3) a caudal part, which, in the female, is transformed into the urethra and the 

 vestibule of the vagina, whilst in the male it is developed into the urethra. 



Derivatives of the Stomatodaeum. When the stomatodseum is first definitely 

 established, it is bounded cranially (anteriorly) by the caudal surface of the 

 ventrally bent terminal part of the head, caudally by the conjoined ventral ends of 

 the mandibular arches, and laterally by the dorsal parts of the mandibular arches, 

 and the maxillary processes, which grow ventrally from the dorsal parts of the 

 mandibular arches. The space is open ventrally, and it is closed dorsally by the 

 bucco-phaTyngeal membrane, which separates it from the fore-gut (Fig. 55). 



