620 REPRODUCTION. 



In the anterior portion of the area pellucida, the substance of the 

 blastoderm rises up in such a manner as to form two nearly parallel 

 ridges or plates, which approach each other from side to side, over what 



will be the dorsal aspect of the embryo, 



FlG - 177 - and are therefore called the "dorsal plates." 



Between them is included a groove, 

 termed the "medullary groove." The 

 dorsal plates afterward meet and coalesce 

 on the median line, thus converting the 

 intervening groove into a canal. The 

 coalescence of the dorsal plates takes 

 place first in the anterior part of the area 

 pellucida, extending thence gradually back- 

 ward ; and when it is complete the whole 



Diagrammatic view of the IMPREG- of the medullary groove becomes a closed 

 NATED EGG, showing the embryonic canal. This is the "medullary canal;" 



spot, area pellucida, and primitive ^ within ^ jg formed ^ ccrebro . gpinal 



axis, by a growth of nervous matter 



from its internal surface. At its anterior extremity, the medullary 

 canal is large and rounded, producing the brain and medulla oblongata ; 

 its remainder is narrow, and pointed posteriorly, corresponding in form 

 with the future spinal cord. At the same time, the thickened edges of 

 the blastoderm grow outward and downward, extending over the lateral 

 portions of the vitelline mass. They are called the " abdominal plates ;" 

 and they approach each other below enclosing the abdominal cavity, as 

 the dorsal plates above enclose the medullary canal. At last they unite 

 on the median line, embracing the whole of the internal blastodermic 

 layer, which encloses in turn the remains of the vitellus and the albu- 

 minous fluid contained in its cavity. 



Simultaneously with these changes, there is formed, in the thickened 

 central part of the blastoderm, immediately beneath the medullary 

 canal, a longitudinal, cylindrical cord the "chorda dorsalis." Around 

 the chorda dorsalis are afterward developed the bodies of the vertebrae, 

 the oblique processes of the vertebra running upward into the dorsal 

 plates, while the transverse processes and ribs run outward and down- 

 ward in the abdominal plates, to encircle more or less completely the 

 corresponding portion of the body. 



In a longitudinal section of the egg, during this process, the thickened 

 portion of the external blastodermic layer (Fig. 178, a ) may be seen in 

 profile. The anterior portion (. 2 ), which will form the head, is thicker 

 than the posterior ( 3 ), which will form the tail. As the whole mass 

 grows rapidly, in both the anterior and posterior direction, the head 

 becomes thick and voluminous, while the tail begins to project back- 

 ward, and the egg assumes an elongated form. (Fig. 179.) The abdom- 

 inal plates also meet upon its under surface, and complete the closure of 

 the abdominal cavity. The internal blastodermie layer is embraced by 

 the abdominal plates, enclosing, as before, the remains of the vitellus. 



