The Atlantic Slope Naturalist. 



Vol. 1. 



NARBERTH, PA., MARCH AND APRIL, 1903. 



No. 1. 



A BRIEF REVIEW OF THE EMBRY- 



OLOGICAL FORMATION OF THE 



CENTRAL NERVOUS SYSTEM. 



By Frank H. Wldman, M. D., 



Lecturer on Embryology, in the Hahnemann Med- 

 ical College of Philadelphia. 



The formation of the central nerv- 

 ous system takes place in a tolerably 

 uniform manner in Reptiles, Birds, and 

 Mammals. The"process has been quite 

 thoroughly worked out in the study of 

 the development of bird embryos ; the 

 chick, owing to the comparative ease 

 with which the embryos can be ob- 

 tained at any desired age, has become 

 a classic in the study of embryonic 

 development, and will serve as the 

 basis for the following description : 



The embryonic fundament of the 

 central nervous system is known as the 

 Neural Canal. This canal extends, 

 as a hollow tube, along the entire dor- 

 sal surface of the circumscribed area 

 in which the body of the embryo will 

 develop. The development of the 

 Canal commences very early, in fact 

 it is one of the very first of the embry- 

 onic structures to become differenti- 

 ated ; it is developed from the outer of 

 the three primary germ layers, the 

 Ectoderm — the remaining two layers 

 — the Mesoderm and the Entoderm— 

 taking no part in the formation of the 

 neural canal. 



The first differentiation of the Ecto- 

 derm to form the neural canal is the 

 formation of a flat plate of cells, 

 which is known as the Medullary 

 Plate. This plate extends from the 

 head end to the tail end of the devel- 

 oping embryo along its dorsal aspect, 

 and is continuous on either side with 

 the remaining portion of the Ectoderm 



from which it is formed. By a de- 

 pression of the central portion of this 

 flat Medullary Plate, and by an up- 

 ward growth of its edges, the plate is 

 converted into a shallow groove which 

 is known as the Medullary Groove, the 

 upward growing edges being known as 

 the Medullary Folds. These medul- 

 lary folds finally unite along their 

 edges and over the dorsal surface of 

 the embryo, thus forming a closed 

 tube, whose walls are composed of 

 several layers of epithelial cells. This 

 tube is the Neural Canal, and from 

 this simple tube the entire nervous 

 system is developed. The ectoderm 

 from which the Neural Canal was 

 formed, later grows over the closed 

 canal, all connection between the two 

 being severed ; in that way the Canal 

 comes to lie under the surface. The 

 union of the Medullary Folds takes 

 place first at the future cervical region 

 of the embryo, the closure proceeding 

 from that point both forward and back- 

 ward ; in the chick the closure com- 

 mences as early as the second day of 

 incubation. In the Mammalia the 

 process of closing commences some- 

 what later, in the Rabbit, for ex- 

 ample, the closure does not commence 

 until the ninth day of development. 



The next step in the development 

 of the nervous system is the differenti- 

 ation of the Neural Canal into the 

 brain and spinal cord. At a very 

 early period in birds, even before the 

 Canal is everywhere a closed tube, 

 there begins to be a noticeable change 

 in the lumen of the Canal, approxi- 

 mately the anterior half of the Canal 

 undergoes a dilation, this dilated por- 

 tion ultimately forming the brain, 



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