234 EMBRYOLOGICAL TYPES 



The urino-genital ducts develop much as in the chick, 

 except that the right oviduct persists, and the testis descends 

 into the scrotum. 



From the fact that the perforation of the mouth does not 

 occur at the extreme front end, but in the centre of the oral 

 membrane, a small pocket is formed morphologically in front 

 of the mouth. This is the so-called preoral gut. In a similar 

 way, a post- anal gut is left after perforation of the anus. 



In the region of the pharynx, the gill-pouches arise as 

 outpushings from the gut to the ectoderm. They do not, 

 however, become perforated. 



Several structures enter into the formation of the diaphragm. 

 The transverse septum moves backwards a considerable 

 distance during development, and it is followed in its course 

 by the phrenic nerve. The transverse septum forms the 

 ventral portion of the diaphragm, and the wall which separates 

 the pericardium from that part of the perivisceral cavity into 

 which the lungs extend. The dorsal portion of the diaphragm 

 separates this pleural ccelom from the abdominal cavity behind, 

 and it is formed by the growth of the mesenteries associated 

 with the liver (which enlarges), kidneys, lungs, and gut. 



As the placenta and the embryo increase in size, the uterus 

 becomes enlarged to accommodate them. This is effected 

 by a great increase in the size of the smooth muscle-cells of 

 which the wall of the uterus is composed, without any increase 

 in their number. 



When the period of gestation is accomplished, the amnion 

 breaks and the embryo is expelled by the contractions of the 

 muscular walls of the uterus. The umbilical cord is torn. 

 The placenta also becomes detached from the wall of the uterus, 

 and, together with clots of blood and debris, is expelled as 

 the after-birth. 



Hair. — The development of hair starts by a thickening 

 of the deeper layer of the epidermis, and its downgrowth into 

 the dermis forming a little cylinder. At its base a papilla is 

 formed, and just above this, the epidermal cells proliferate and 

 give rise to the shaft of the hair. This elongates as more 

 material is added to it from beneath, and it finally emerges 



