BIRDS. 353 



long- direction of the egg, but lies transversely upon the yolk, and 

 making nearly a right angle with the long axis of the egg. The 

 primitive streak is a groove; a few hours later there appears at its 

 two sides an ascending ridge-like dorsal lamina. These two dorsal 

 laminge, which gradually approach each other above and at length 

 coalesce in the mid-plane, were named by Pander the primitive 

 folds {Primitw-f alien) . In the primitive streak, and so between 

 these dorsal lamina3, the central nervous system is formed; beneath 

 it arises as a fine thread the notochord (chorda dorsalis, see above, 

 p. 8), the first commencement and precursor of the spinal column; 

 afterwards this chord becomes surrounded by the bodies of the 

 vertebrae, and finally is quite suppressed by them. On each side 

 of the cliorda dorsalis small four-sided white plates begin to be 

 distinguished in the dorsal laminae, at first four or five, in front of 

 and behind v/hich others gradually become visible ; these little plates 

 are usually considered to be the commencement of the vertebrae ' ; 

 the mucous lamina is loosely attached to the inferior surface (that 

 turned towards the yolk) of this first commencement of the chick. 

 As yet there is no abdominal cavity. The commencement of it is 

 made, on the second day, by two parts of the animal layer situated 

 on the outside of the dorsal laminas, the so-named {lamince. ven- 

 f rales), which extend superficially and bend downwards. These 

 laminge thus form the lateral walls of the abdominal cavity, which 

 is slowly closed, and not completely so before the development of 

 the chick has been completed. The cavity, on the contrary, which 

 contains the central parts of the nervous system, the spinal marrow 

 and the brain, is speedily inclosed. Even on the second day the 

 dorsal laminte meet by their margins in the mid-plane and coalesce. 

 In front the dorsal lamince separate farther from each other, and 

 thus bound a larger space which incloses a clear fluid situated in ve- 

 sicular spaces behind each other, from which afterwards the cerebral 

 mass is developed. Already, on the first day, the embryo has 

 assumed a curved direction, and the anterior extremity (originally 

 the broader end of the 7iofa primitiva) is bent more and more down- 

 wards. At the posterior extremity, on the second day also, there 

 is a curvatm-e, and thus the embryo lies, like an upset boat, with 



1 Eemak, on tlie contrary, thinks these plates may be regarded as rudiments of the 

 cerebro-spinal nerves, Mueller's Archiv, 1843, s. 478 — 484. 



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