CHAP. XLI.] VENTRAL LAMINAE. 587 



and inner part only. About this time a few small square-shaped 

 plates make their appearance in the central portion of each dorsal 

 lamina. These plates are the first rudiments of the vertebrae. 

 The soft chorda dorsalis, a structure which exists permanently in the 

 lower cartilaginous fishes, appears subsequently between the rows 



fig. 277. 



Plan shewing the mode of development of the dorsal and ventral laminae. . Serous or 

 animal layer; e. mucous or vegetative layer: 6. chordt dorsalis; c. ventral lamina, arching 

 downwards to enclose the intestines, the rudiment of which is shown at </.; /. dorsal lamina 

 arching over to convert the groove into a canal for the spinal cord ; g. aorta. 



of plates, but this is by no means so distinctly marked in the mam- 

 malian ovum as it is in the embryo frog or fish, while in the lowest 

 cartilaginous fishes, as Fig 278 



the myxine, the lam- 

 prey, and others, it is 

 persistent. The two 

 plates of opposite sides 

 gradually approximate, 

 and ultimately coa- 

 lesce, including be- 

 tween them a portion 

 of the chorda dorsalis, 

 a temporary structure, 

 which disappears en- 

 tirely without being- 

 transformed into any 

 more permanent tex- 

 ture. 



The lamina viscerales 

 or ventral lamina, are 



also developed in the Germinal membrane with rudiments of embryo of the dog. After 



r Bischoff. . Primitive groove, not closed; b. the three d.Uta- 



SerOUS laver, and COn- tions corresponding to the three vesicles of the cerebrum ; c. space 



" at lower part of groove or sinvs rliomboidafu. The streak at the 



tinilOUS With the la- bottom of the groove is the chorda dorsalis. 



mina3 dorsales. They 



