ECTODERMAL DERIVATIVES 99 



and there has a nucleus. Investigators are not in agreement as to the 

 origin of the neurilemma. 



The development of the ventral nerve cord is similar in insects gen- 

 erally, including the apterygotes. Some minor differences to be noted 

 are the following: in Xiphidium Wheeler (1893) found four neuroblasts 

 in transverse section of each lateral cord; in Locusta (Roonwal, 1937) 

 there are four or, rarely, five; in Euryope (Paterson, 1932) there are 

 three in the abdomen but four in the thorax; in Forficula (Heymons, 

 1895a) the number is variable; in Pieris (Eastham, 1930a) there are three 

 at first. Baden states that in Melanopltis the neuroblasts increase trans- 

 versely from one to three in the narrower interganglionic portions of each 

 lateral cord and from three to five in the intraganglionic portions. In 

 this form also in each ganglia caudally, from the mandibular to the tenth 

 abdominal inclusive, a typical neuroblast is differentiated. On the other 

 hand, in Xiphidium, Locusta, and Apis one median cord neuroblast dif- 

 ferentiates in each intersegmental region, and in Forficula (Heymons, 

 1895a) there are several in this region. Nerve cells (the daughter cells) 

 are formed by unequal division of the large neuroblasts in Gryllotalpa 

 (Korotneff), Xiphidium (Wheeler), Forficula (Heymons) Eutermes 

 (Strindberg), Pieris (Eastham), and Musca (Escherich), but subsequent 

 division does not occur. Subsequent division of daughter cells occurs in 

 Doryphora (Wheeler), Apis (Nelson), Calandra oryzae (Tiegs and Mur- 

 ray), and other embryos. 



The total number of nerve ganglia belonging to the ventral chain 

 in insects differs with the species. During the earlier stages in the 

 development in many of the more primitive insects as well as some of the 

 more highl.y specialized ones, there are 17, one for each segment from 

 the mandibular to the eleventh abdominal inclusive. This is true for 

 Lepisma, Gryllotalpa, Periplaneta, Gryllus, Locusta, Leptinotarsa, Donacia, 

 Calandra, Hylotoma, Chalicodoma, Apis, and others. In numerous 

 insects, however, the seventeenth, i.e., the eleventh abdominal, is lacking. 

 As development proceeds there is a tendency for some of the ganglia, 

 especially the two or three posterior ones, to fuse. Extreme consolida- 

 tion (cephahzation) occurs in the larvae of the higher Diptera, scarabaeid 

 beetles, etc., in which the entire ventral nerve cord, including the sub- 

 esophageal ganglion, is consolidated into a single elongate mass lying 

 in the anterior part of the thorax. 



The development of the median cord (Fig. 46, 7nst), unlike that of the 

 lateral cords, appears to differ considerably in different insects. That 

 it is derived from the median strip of ventral ectoderm, forming the roof 

 of the neural groove, seems to be certain. The ultimate fate of this strij) 

 is less uniform. Nelson (1915) states that all investigators of the 

 subject — with the exception of Wheeler^ — agree with Hatschek that the 



