530 



THE DEVELOPMENT OF THE INDIVIDUAL 



cavity. These layers come together both above and below the digestive 

 tube forming the mesenteries which hold these organs in place. 



Formation of the Organ Systems 



Brain and Spinal Cord. The following descriptions are based largely 

 on the embryology of the frog, since this embryo is large and easily seen 

 and it is not covered by a shell or extraembryonic membranes. The 

 early stages of the embryo are round or nearly so, but as development 

 proceeds the embryo becomes elongated in an anterior-posterior direc- 

 tion. At the same time the ectodermal cells on the dorsal surface divide 

 rapidly and form a plate of cells known as the neural plate. The sides 

 of this plate rise up and form a pair of neural folds leaving a depression 

 between them, the neural groove. The folds soon close and form a 



Midgut 



Spina 



Hindcjut 



Dorsal lip 

 of blastopor 



Proctode 

 (to form anus) 



ochord 

 ndbrain 



Midbrain 



Mesoderm Yolk f heart 



(endoderm) 



Stomodeum 

 (to form mouth) 



Fig. 33.6. Longitudinal section through a typical vertebrate embryo showing the 

 method of formation of some of the body organs. 



hollow neural tube. The anterior end of this tube is destined to form the 

 brain. It gradually becomes constricted so as to form the three main 

 parts, the forebrain, the midbrain, and the hindbrain. The posterior 

 portion of the tube makes the spinal cord. 



Alimentary Canal. The alimentary tract develops from the archen- 

 teron or primitive gut. When the neural groove closes over, the blasto- 

 pore is temporarily covered. A posterior invagination, however, forms 

 where the ectoderm and endoderm at first fuse and then perforate and 

 form the anus. This invagination is known as the proctodeum. Occa- 

 sionally this opening fails to form during embryological development 

 and a surgeon must correct this mistake of nature. At the anterior end 

 of the embryo, the opening for the mouth is produced by the ectoderm 

 and endoderm fusing and forming the stomodeum. As the opening for 



