DEVELOPMENT OF THE NERVOUS SYSTEM. 



lymphatics, at their first formation, as in their perfect state, are distin- 

 guished from blood-vessels by the rarity of their anastomoses. 



Development of the Nervous System* 



First Traces of the Nervous System. As was before observed (page 84) 

 the account given by Reichert of the original formation of the central 

 parts of the nervous system has been considerably modified by the investi- 

 gations of Bischoff on the development of Mammalia.f The two oval 

 masses or laminae, between which the primitive groove is situated, are not, 

 as was shewn also by Von Baer, the parts out of which the nervous system 

 alone is formed, but are for the most part merely the foundations for the 

 formation of the dorsal part of the trunk of the body, the central nervous 

 system being developed only from that portion which immediately borders 

 upon the primitive groove. Concerning the time at which this development 

 first presents itself, Bischoff offers a somewhat different account to that 

 given by Von Baer, who was of opinion that it did not commence until 

 after the conversion of the groove into a canal by the junction of the 

 lateral masses on each side of it. But Bischoff states that previous to 

 the formation of a canal, nervous substance is developed along the whole 

 inner surface of the groove, and apparently by a metamorphosis of the 

 portions of the lateral masses immediately contiguous to the groove. The 

 substance composing these portions gradually assumes a pellucid aspect 

 like nervous substance, and increases in quantity : the inner border of each 

 mass thus altered, then approximates and gradually unites with its fellow 

 of the opposite side, so as to convert the previous groove into a tube, the 

 walls of which thus consist of nerve-substance, while the hollow axis con- 

 stitutes the central canal of the spinal cord. The approximation and 

 union of the margins of the groove takes place first about the middle of 

 the groove, and then proceeds upwards and downwards from this point. 

 At the commencement of this union at the middle part of the groove, the 

 upper or cephalic is formed (as was shewn also by Reichert in the chick)J 

 into three successive dilatations which are the vesicles from which the 

 brain is formed. At the opposite or caudal extremity the groove presents 

 a lancet-shaped dilatation : this corresponds to the future Cauda equina 

 (or Sinus rhomboidalis, as it is named in birds, vide fig. 11, p. 84). 

 That the two oval masses bounding the primitive groove do not con- 

 stitute the rudiments of the central parts of the nervous system, is shewn 

 to be the case in Amphibia also, from the result of Vogt's investigations 

 on the development of the toad.|| In this amphibious animal, he finds, 

 as Bischoff has found in Mammalia, that the central nerve-substance is 



* Miiller's Physiology, p. 1627. 



t Entwickelungs-gesch. der Stiugeth. und des Menschen, p. 165, et seq. 



| Miiller's Physiology, p. 1547. || Entwickelungs-gesch. der Geburtshelferkrote, p. 66. 



