FORMATION OF ORGANS. 



its turn the remains of the original vitellus and the albuminous 

 fluid which has accumulated in its cavity. 



Fig. 196. 



Fig. 197. 



Transverse section of EGO in an early 

 stage of development. 1. External layer 

 of blastodermic membrane. 2, 2. Dorsal 

 plates. 3. Internal layer of blastodermic 

 membrane. 



IMPREOX ATED ERG, at a somewhat 

 more advanced period. 1. Umbilicus, or 

 point of union between abdominal plates. 

 2, 2. Dorsal plates united with each other 

 on the median line and inclosing the spinal 

 canal. 3. 3. Abdominal plates. 4. Sec- 

 tion of spinal column, with lamine and 

 ribs. f>. Internal layer of blastodermic 

 membrane. 



During this time, there is formed, in the thickness of the external 

 blastodermic layer, immediately beneath the spinal canal, a longitu- 

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

 chorda dorsalis are afterward developed the bodies of the vertebrae 

 (Fig. 197, 4), forming the chain of the vertebral column; and the 

 oblique processes of the vertebrae run upward from this point into 

 the dorsal plates ; while the transverse processes, and ribs, run out- 

 ward and downward in the abdominal plates, to encircle more or 

 less completely the corresponding portion of the body. 



If we now examine the egg in longitudinal section, while this 

 process is going on, the thickened portion of the external blasto- 

 dermic layer may be seen in profile, as at i , Fig. 198. The anterior 

 portion (*), which will form the head, is thicker than the posterior 

 (3), which will form the tail of the young animal. As the whole 

 mass grows rapidly, both in the anterior and the posterior direc- 

 tion, the head becomes very thick and voluminous, while the tail also 

 begins to project backward, and the whole egg assumes a distinctly 

 elongated form. (Fig. 199.) The abdominal plates at the same time 



