144 



THE FOURTH DAY. 



[CHAP. 



Fig. 4 ; 



Section through the Lumbar Region of an Embryo at the End of the 



Fourth Day. 



n. c. neural canal, p. r. posterior root of spinal nerve with ganglion, a. r. anterior 

 root of spinal nerve. A. G. C. anterior grey column of spinal cord. A. W. C. 

 anterior white column of spinal cord just commencing to be formed, and 

 not very distinctly marked in the figure, m. p. muscle plate, ch. notochord. 

 Tr. R. Wolffian ridge. A. 0. dorsal aorta. V. c. a. posterior cardinal vein. 

 W.d. Wolffian duct; its section is not circular, owing to its being cut 

 through at a point where it is being joined by one of the tubules. W. b. 

 Wolffian body, consisting of tubules and Malpighian corpuscles. One of 

 the latter is represented on each side, that on the left hand having its 

 glomerulus entirely filled with blood corpuscles, g. e. germinal epithelium. 

 M. d. commencing involution of germinal epithelium to form the duct of 

 M tiller, d. alimentary canal. M. commencing mesentery. S. 0. somato- 

 pleure. S. P. splanchnopleure. V. blood-vessels, pip. pleuroperitoneal 

 cavity. 



