DEVELOPMENT OF THE EMBRYO. 



637 



Fig. 235. 



CHAPTER XIV. 



DEVELOPMENT OF THE EMBRYO NERVOUS SYSTEM 

 ORGANS OF SENSE, SKELETON, AND LIMBS. 



THE first trace of a spinal cord in the embryo consists of the 

 double longitudinal fold or ridge of the blastodermic membrane, 

 which shows itself at an early period, as above described, on each 

 side the median furrow. The two laminae of which this is com- 

 posed, on the right and left sides (Fig. 235, a, b), unite with each 

 other in front, forming a rounded dilatation (c), 

 the cephalic extremity, and behind at d, forming 

 a pointed or caudal extremity. Near the pos- 

 terior extremity, there is a smaller dilatation, 

 which marks the future situation of the lumbar 

 enlargement of the spinal cord. 



As the laminae above described grow upward 

 and backward, they unite with each other upon 

 the median line, so that the whole is converted 

 into a hollow cylindrical cord, terminating ante- 

 riorly by a bulbous enlargement, and posteriorly 

 by a pointed enlargement; the central cavity 

 which it contains running continuously through 

 it, from front to rear. 



The next change which shows itself is a divi- 

 sion of the anterior bulbous enlargement into 

 three secondary compartments or vesicles (Fig. 

 236), which are partially separated from each other by transverse 

 constrictions. These vesicles are known as the three cerebral vesi- 

 cles, from which all the different parts of the encephalon are after- 

 ward to be developed. The first, or most anterior cerebral vesicle 

 is destined to form the hemispheres; the second, or middle, the 

 tubercula quadrigemina ; and the third, or posterior, the medulla 

 oblongata. All three vesicles are at this time hollow, and their 



Formation of CERE- 

 BRO-SPINAL Axis. 

 o, b. Spinal cord. c. Ce- 

 phalic extremity. d. 

 Caudal extremity. 



