THE EMBRYO. 



1209 



rectum behind, and the Wolffian and Mlillerian ducts now open into the urogenital 



sinus. 



The urinary bladder, as before stated, is formed by a dilatation of a part of the 

 mtra-embryonic portion of the allantois. At the end of the second month the 

 middle part of this portion of the allantois becomes dilated into a spindle-shaped 

 cavity, which persists as the urinary bladder. Between the lower extremity of 

 the spindle-shaped dilatation and the intestine is the urogenital sinus, into which 

 the Miillerian and Wolffian ducts now open, and which becomes the first part of 

 the urethra. The upper part of the intra-embryonic portion of the allantois, 

 which is not dilated, forms the urachus (Fig. 776) ; this extends into the umbilical 

 cord, and at an early period of embryonic existence forms a tube of communica- 

 tion with the allantois. It is obliterated before the termination of foetal life, but 

 the cord formed by its obliteration is perceptible throughout life, passing from the 

 upper part of the bladder to the umbilicus. It occasionally remains patent after 

 birth, constituting a well-known malformation. 



The suprarenal bodies are developed from two different sources. The medul- 

 lary part of the organ is of epiblastic origin, and is derived from the tissues form- 

 ing the sympathetic ganglia of the abdomen, while the cortical portion is of 

 mesoblastic origin, and originates as an outgrowth from the upper part of the 

 Wolffian body. The two parts are at first quite distinct, but become combined in 

 the process of development. The suprarenal capsules are at first larger than the 

 kidneys; about the tenth week they equal them in size, and from that time 

 decrease relatively to the kidney, though they remain, throughout foetal life, 

 proportionally much larger than in the adult. 



o. cornea, t. inues. a. vj varies. K. jvcummnui - ,, ^..K^O A TK 



foetus of three months : a. The body of the uterus. 6. The round ligament, c. The Fallopian tubes, d. The 

 ovaries, e. Remains of the Wolffian bodies. 



Ovaries and Testicles. The first appearance of the reproductive organs is 

 essentially the same in the two sexes, and consists in a thickening of the epithelial 

 layer which lines the peritoneal or body-cavity close to the inner side of the 

 Wolffian body. Beneath this thickened epithelium an increase in the mesoblast 

 takes place, forming a distinct projection or ridge. This is termed the genital 

 ridge (Fio-. 774), and from it the testicle in the one sex, and the ovary in the other 

 are developed. As the embryo grows the genital ridge gradually becomes pinched 

 off from the Wolffian body, with which it is at first continuous, though it 

 remains connected to the remnant of this body by a fold of peritoneum, t 

 mesorchium or mesovarium. About the seventh week the distinction ot sex begin 

 to be perceptible. 



