Comparative Development. 43 



ment, and bow all that pass a certain stage of development, continue 

 their embr} r omc growth by feeding upon a bag of nutritive } 7 elk. As- 

 tonishing as these correspondences are, the employment of the amnion 

 and the placenta in the embryonic development of vast tribes of verte- 

 brates, is yet more wonderful. 



FIGS, 64, 65. Diagrammatic, vertical and horizontal sections of vertebrate brain. 



Cb. Cerebellum. 



I. Olfactorii. Il.-Optici. 



FM. Foramen of Munro. CS.- 



CQ. Corpora qnadrigemina. CC. Crura cerebri. 



Pv. Pons varolii. MO. Medulla oblongata. 



III. Point of exit of the motores oculorum. 



IV. ' pathetic!. 



VI. ' abducentes. 



V-XIL Origins of the other cerebral nerves. 



1. Olfactory ventricle. 2. Lateral ventricle. 3. 3d ventricle. 4. 4th ventricle- 



+. Iter a tertio ad quartum ventriculum, or aqueduct of sylvius. 



The heavy line in fig. 64, running from PN to Py, is lamina terminalis. 



The space above the pituitary body or gland, and below fig. 3, is the infundibulum. 



(After Huxley.} 



The amnion and its manner of growth and use in inclosing and pro- 

 tecting the delicate embr} T o, has been described. The embryo of rep- 

 tiles and birds, as well as of all mammals, are nursed in this amnion 

 sac. It is impossible to overrate the importance of this astonishing 

 fact in proving the blood relationship of birds, reptiles and mammals, 

 and their descent from a common amnion bearing ancestor. 



The formation of the placenta from the ancient and primitive urinary 

 bladder, points out another but later family characteristic. 



This primitive urinary sac is a blind sac growing from the front or 

 belly side of the intestine near its hind end, and first appears as a per- 

 manent organ in the adult dipneusta ( lepidosiren, &c.), and is an em- 

 bryonic organ in the amphibia and all mammals, including man. In 

 all the mammal embyros this urinary sac enlarges into the allantois or 



