ECTODERMAL DERIVATIVES 101 



in some insects the median cord may form a more or less continuous 

 median nerve and is perhaps a primitive condition. 



In Apis Nelson (1915) found that the origin and fate of the median 

 cord conforms to Grassi's account and is also similar to that of Hydro- 

 philus (Graber, 1890). In both forms the median cord is united with the 

 epidermis in the interganglionic region but is independent in the region 

 of the ganglion. Here, nevertheless, these interganglionic spaces are, 

 up to the time of hatching, of very slight extent in an anteroposterior 

 direction, and the anterior and posterior commissures are close together, 

 so that it is obvious that the anterior and posterior median gangliomeres, 

 as well as the central gangliomeres, are formed from the median cord. 

 The term " gangliomere " as used here designates each of the ganglionic 

 regions of a ventral nerve ganglion — two lateral regions and one in front, 

 one between, and one behind the commissures. 



Tiegs and Murray (1938) state that the median cord in Calandra 

 oryzae contributes to the formation of the ganglia; its intersegmental 

 neuroblasts attach themselves to the posterior wall of the ganglia, and 

 the intrasegmental (neurogenic) cells form the roof of the completed 

 ganglia, their axons contributing to the formation of the transverse 

 commissures and longitudinal connectives. Baden (1936, 1937), on the 

 other hand, maintains that in Melanoplus differentialis all the fibers 

 of the commissures seem to come from the ganglion cells alone and that 

 the median cord does not differentiate into any tissue but apparently 

 degenerates and is absorbed. 



The origin of the neurilemma in insects has been the subject of con- 

 troversy. Nusbaum (1883) maintained that this tissue in the roach 

 owes its origin to cells derived from median cord elements that grow 

 around the ventral nerve cord as a cellular membrane. A second mem- 

 brane from the same source gives rise to an inner neurilemma between 

 the cortical cellular layer and the neuropile strands in each half of the 

 ventral cord. Since Nusbaum (1886) held that the median cord is derived 

 from the provisional yolk-cell membrane which he stated is of yolk-cell 

 origin, the neurilemma itself must therefore be a yolk-cell derivative. 



In Forficula Heymons (1895a) found that at the time when the 

 neuroblasts begin to disappear the surface of the ganglion is being formed 

 by occasional cells in the ganglion which flatten and spread out to form 

 the outer neurilemma. The inner neurilemma is formed in a similar 

 manner — flattened cells that spread out between the cortical layer and the 

 neuropile. Heymons concluded that the outer neurilemma apparently 

 arises from cells that during the segregation of the neuroblasts from 

 the dermatogenous layer were separated off from the latter. Wheeler 

 (1893) believed that it arose from the intraganglionic sections of the 

 median cord in Xiphidium. 



