No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 8/ 



from mouth to anus, the latter constitute an interrupted series 

 between the same two points. They are single, isolated cells, 

 which occur only intersegmentally. That such is their distribu- 

 tion may be distinctly seen in frontal sections like the one rep- 

 resented in Fig. 30. This section passes through the first 

 to fifth abdominal segments at the level of the median cord 

 neuroblasts {mnb), which are seen to lie distinctly between 

 the segments, where the walls of the neural furrow dilate 

 at intervals for their accommodation. At first the daughter- 

 cells are given off in the same direction as those of the lateral 

 cords, but soon the triangular space to which they are confined 

 will no longer contain the older cells of the series and these 

 are pushed along the floor of the neural furrow. This produces 

 an angular flexure in the cell-column, but later the whole mass, 

 including the neuroblast, assumes a horizontal position. This 

 change in the position of the median cell-mass is seen to have 

 taken place in the median sagittal section from an embryo in 

 Stage G (Fig. 29). The neuroblast iinnb) is in each segment 

 directed caudad, while the mass of small and deeply stainable 

 daughter-cells {vig) is wedged in under the commissures. The 

 section passes through the sub-oesophageal ganglion, which 

 consists of the fused ganglia of the mandibular and both maxil- 

 lary segments {md. g; vix. g^ ; inx. g^), and through the pro- and 

 mesothoracic ganglia (p.g' ; /. g^). Transverse furrows (/. g' ; 

 z.g^), which I shall consider later, separate the unfused ganglia 

 from one another, and as the median cord cells lie in front of 

 these furrows, they must be regarded as belonging not to the 

 intersegmental region of the ectoderm, but to the posterior 

 portions of the separate ganglia. Each ganglion possesses a 

 median cord neuroblast, so that, beginning with the mandibular, 

 which is the first ganglion in the nerve-cord proper, and ending 

 with the tenth abdominal, there are in all sixteen median 

 mother-cells. Each of these, after producing its quota of 

 ganglionic elements, deteriorates in the same way as the 

 mother-cells of the lateral cords. 



The development of the Punktsubstanz may be readily fol- 

 lowed in Xiphidiwn. It arises in each ganglion as two separate 

 masses. Each of the daughter-cells of the lateral neuroblasts 



