60 THE CHANGES PROM STAGE A TO F. 



separation between the cerebral rudiments and the ventral 

 cords? 



There can be no doubt whatever that in Peripatus 

 capensis there is no such break at any stage of develop- 

 ment. A study of transverse sections of many different 

 embryos from Stages c — e conclusively demonstrates that the 

 ventral nerve-cords, which are developments of the inner 

 portions only of the lateral thickenings, are continuous at 

 all stages of their existence, both before and after their 

 separation from the ectoderm, across the boundary between the 

 first and second somites with the inner portion of the cerebral 

 rudiment, which is a development of the whole surface of the 

 pre-oral part of the lateral thickening. It is true that at first 

 this part of the central nervous system is weaker than the parts 

 in front and behind it, but long before the separation from the 

 ectoderm occurs (Plate VII, fig. 196; Plate VIII, fig. 35, c.o.w.), 

 it forms a well-marked cord with a layer of white matter. 

 The whole central nervous system of Peripatus 

 capensis develops, therefore, as a continuous struc- 

 ture from the ectoderm, and the independent origin and 

 secondary connection of the cerebral ganglia and ventral chain, 

 which has been asserted for some Arthropod a, e.g. for Spider* 

 by Balfour (No. 19), for Annelida by Salensky (No.44),Lum- 

 bricus by Kleinenberg (No. 31), and for Mollusca by various 

 observers, does not hold for Peripatus.^ Balfour held the same 

 view with regard to this point in Peripatus (No. 18, p. 337). 



I entirely agree with the remarks of Hatschek (No. 28, p. 8) 

 on this subject. He holds, in opposition to Salensky, that the 

 circumoral part of the nervous system in the annelid larvae 

 which he has investigated, develops fron the ectoderm in con- 

 tinuity with the apical ganglion and ventral nerve-cords; and 

 I am strongly inclined to think that a further and closer inves- 

 tigation will show the same fact to hold for other Annelids and 

 Molluscs. 



' Kennel apparently holds the opposite view on this point. It is difficult 

 to believe that the West Indian Peripatus differs in this respect from the 

 Cape species. 



