826 EMBRYOLOGY 



forms most of the mass of the bone ; it disappears at the eighth month " 

 (Meckel). This structure is known as the cartilage of Meckel. 



There are no special points for description in the development of the 

 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 gan- 

 glionic matter and constitute the olfactory bulbs, their pedicles being the 

 so-called olfactory nerves, or olfactory commissures. 



So 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 

 on 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 bulb when 

 the placental circulation is interrupted. 



DEVELOPMENT OF THE DIGESTIVE SYSTEM/ 



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 extremity 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 plate 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 communication between the lower 

 part of the intestinal tube and the allantois, which forms the canal 

 known as the urachus ; but that portion of this communication which 

 remains enclosed in the abdominal cavity becomes separated from the 

 urachus, is dilated and eventually forms the urinary bladder. When 

 the bladder is shut off, it communicates with the lower portion of the 

 intestine, which is called the cloaca ; but it finally loses this connection 

 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 develop- 

 ment, a portion of the intestine protrudes at the umbilicus, where the 



