822 GENEKATION. 



olfactory lobes, which is very simple. These are offshoots from the first 

 cerebral vesicle, appearing at the inferior and anterior part of the cerebral 

 hemispheres, a little later than the parts connected with vision and audition. 

 The vesicles themselves become filled with ganglionic matter and constitute 

 the olfactory bulbs, their pedicles being the so-called olfactory nerves, or 

 olfactory commissures. 



As far as the action of the nervous system of the foetus is concerned, it 

 is probable that it is restricted mainly to reflex phenomena depending 

 upon the spinal cord, and that perception and volition hardly exist. It is 

 probable that many reflex movements take place in utero. When a foetus is 

 removed from the uterus of an animal, even during the early months of 

 pregnancy, movements of respiration occur ; and it is well known that efforts 

 of respiration sometimes take place within the uterus. These are due to the 

 want of oxygen-carrying blood in the medulla oblongata when the placental 

 circulation is interrupted. 



DEVELOPMENT OF THE DIGESTIVE APPABATUS. 



The intestinal canal is the first formation of the digestive system. This 

 is at first open in the greatest part of its extent, presenting, at either extrem- 

 ity of the longitudinal gutter, in front of the spinal column, a rounded, blind 

 extremity, which is closed over in front for a short distance. The closure of 

 the visceral plates then extends laterally and from the two extremities of the 

 intestine, until only the opening remains for the 

 passage of the umbilical cord and the pedicle of the 

 umbilical vesicle. There is at first an open com- 

 munication between the lower part of the intestinal 

 tube and the allantois, which forms the canal known 

 as the urachus ; but that portion of this communi- 

 cation which remains enclosed in the abdominal 

 cavity becomes separated from the urachus, is di- 

 lated and eventually forms the urinary bladder. 

 When the bladder is first shut off, it communicates 

 with the lower portion of the intestine, which is 



FIG. 30o. Foetal pig^ showing a 



loop of intestine, forming called the cloaca i but it finally loses this connec- 



an umbilical hernia (Dal- . 



ton). tion and presents a special opening, the urethra. 



As development advances, the intestine grows 

 rapidly in length and becomes convoluted. It is 

 held loosely to the spinal column by the mesentery, 



a fold of the peritoneum, this membrane being reflected along the walls of 

 the abdominal cavity. In the early stages of development, a portion of the 

 intestine protrudes at the umbilicus, where the first intestinal convolution 

 appears ; and sometimes there is a congenital hernia of this kind at birth, 

 which usually disappears under the influence of gentle and continued press- 

 ure. An illustration of this is given in Fig. 305. This protrusion, in the 

 normal process of development, is gradually returned into the abdomen, as 



