DEVELOPMENT OF THE IMPREGNATED EGG. 



725 



anterior part of the,, are^pellucida and extends gradually backward , 

 and when it is jtoule)te throughout their length, the whole of the 

 medullary groove^BmbeenjKyiverted into a closed canal. This is the 

 "medullary canal ]^^d in its cavity will afterward be formed the cere- 

 bro-spinal axis, by a growth of nervous matter from its internal surface. 

 At its anterior extremity, the medullary canal is large and rounded, to 

 accommodate the brain and the medulla oblongata; its remainder is 

 narrow, and pointed posteriorly, and is destined to contain the spinal 

 cord. 



In a diagrammatic section of the egg at this stage, made transversely 

 to the longitudinal axis of the embryo (Fig. 242), the dorsal plates may 

 be seen approaching each other above, on each side of the medullary 

 groove. At a more advanced period (Fig. 243) they are fairly united 

 with each other, and inclose the cavity of the medullary canal. At 



Fi?. 242. 



Fig. 243. 



Diagrammatic section of the impregnated 

 EGG in an early stage of development. 1. 

 External blastodermic layer. 2, 2. Dorsal 

 plates. 3. Internal blastodermic layer. 



IMPREGNATED Eoo, at a somewhat 

 more advanced period. 1. Point of union 

 between the abdominal plates. 2, 2. Dor- 

 sal plates united with each other on the 

 median line and inclosing the medullary 

 canal. 3,3. Abdominal plates. 4. Section 

 of the spinal column, with laminae and ribs. 

 6. Internal blastodermic layer. 



the same time, the edges of the thickened portion of the blastoderm 

 grow outward and downward, extending over the lateral portions of the 

 vitelline mass. These are called the "abdominal plates;" and, as they 

 enlarge, they tend to approach each other below and inclose the abdo- 

 minal cavity, as the dorsal plates united above, and inclosed the medul- 

 lary canal. At last the abdominal plates actually unite on the median 

 line (at i, Fig. 243), embracing the whole of the internal blastodermic 

 layer ( 5 ), which incloses in turn the remains of the original vitellus and 

 the albuminous fluid accumulated in its cavity. 



During this time, there is formed, in the thickened central part of 

 the blastoderm, immediately beneath the medullary canal, a longitudinal 

 cartilaginous cord, the "chorda dorsalis." Around the chorda dorsalis 



