GENERAL EMBRYOLOGY 



1619 



Fore-Brain 



Opdc Vesicle 



Miti-Brain 



Hind- Brain 



Spinal Portion o( 

 Neural Tube 



- Mesodermic 

 Somite 



the groove is converted into a tube, which constitutes the neural 

 canal. This canal forms the rudiment of the whole cerebro-spinal 

 nervous system, and it consists entirely of ectoderm. The cephalic 

 end of the canal becomes dilated, and gives rise to the three primary 

 cerebral vesicles, which are the rudi- 

 ments of the fore-brain, mid-brain, 

 and hind-brain, including the cavities 

 •of the cerebral ventricles. The re- 

 mainder of the canal, from the wall 

 of which the spinal cord is developed, 

 persists as the central canal of the 

 spinal cord. As previously stated, the 

 neural canal originally communicates 

 wath the primitive intestinal cavity by 

 means of the neurenteric canal. At 

 this point the ectoderm of the neuraj 

 canal becomes continuous with the 

 entoderm of the primitive intestinal 

 ■cavity. The neurenteric canal, how- 

 ever, is only of temporary duration. 



Notochord or Chorda Dorsalis. — 

 Along the ventral aspect of the neural 

 groove the cells of the entoderm be- 

 come thickened and differentiated to 

 form the notochordal plate, which 

 constitutes the first stage in the for- 

 mation of the notochord from the 

 entoderm. For a description of the 

 notochord, which forms the axis roimd 

 which the vertebral column and its 

 ligaments are developed, see Index. 



Mesodermic Somites. — After the 

 formation of the mesoderm it becomes 

 ■disposed in /o«r parts. Two of these 

 form longitudinal columns, which lie 

 not far from the median or axial line, 

 one being placed on either side of the 

 neural canal and notochord. These 

 two columns constitute the paraxial 

 mesoderm. The other two parts are 

 disposed in the form of sheets, which 

 are known as the lateral plates, each 

 plate lying external to the corre- 

 sponding paraxial column, with which 

 it is at first in continuity. 



The lateral plates of mesoderm are each composed of several strata 

 of cells, which form, on either side, a single compact sheet. Within 

 each plate vacuoles form, which coalesce and so give rise to a cleft- 

 like space. In this space fluid soon acciunulates, and a cavity is 



Fig. 671. — Embryo Chick, 

 SHowaNG Cranial and Spi- 

 nal Portions of Neural 

 Tube and Mesodermic 

 Somites (Kolliker). 



