No. 7. — The Origin and Development of the Central Nervous System 

 in Limax ynaximus. By Annie P. Henchman. 1 



For several years the origin of the central nervous system in Mollusks, 

 both as to method and time of appearance, has been a matter of contro- 

 versy. It has been of especial importance to determine from which of 

 the embryonic layers its parts arise, and to ascertain if its development 

 throws any light on the relations of Mollusks to other important groups 

 of the animal kingdom, particularly Worms. 



Since the observations of the earlier writers, down to about 1874, were 

 carried on without the aid of sections, their conclusions do not merit 

 that degree of confidence which is to be accorded those who have availed 

 themselves of this means of study. 



Most of the later authors agree that the central nervous system arises 

 from the ectoderm, either by an invagination, or by a simple local thick- 

 ening which later becomes detached. However, Bobretzky ('76, pp. 162— 

 169), — the first to use sections, — while conceding that in Fusns there 

 are invaginations of the ectoderm to form the sense organs, concludes 

 that the supra-cesophageal and pedal ganglia arise from the mesoderm, 

 and Biitschli ('77, pp. 227, 228) is inclined to believe that the same is 

 true in Paludina vivipara. 



Von Jhering (74, p. 321) claims for Helix, and both Lankester (74, 

 pp. 382, 383) and Wolfson ('80, pp. 95, 96) for Lymnseus stagnalis, 

 that the central nervous system arises simply from a thickening of the 

 ectoderm. 



Fol ('80, p. 664) has since pointed out, however, that Lankester's con- 

 clusions are based on an erroneous interpretation of cells (" nuchal cells"), 

 which he believes are not at all nervous in their nature. They are the 

 same cells which Wolfson has called the embryonic brain ; but Wolfson's 

 opinion, previously stated, has reference to the definite nervous system, 

 not to this so-called embryonic brain. 



Haddon ('82, pp. 368-370) believes that he has seen the rudiments 

 of the cerebral and pedal ganglia of Nudibranchs in the form of thicken- 



1 Contributions from the Zoological Laboratory of the Museum of Comparative 

 Zoology, under the direction of E. L. Mark, No. XXI. 

 vol. xx.— no. 7. 



