No. 3.] NEUROBLASTS IN THE ARTHROPOD EMBRYO. 339 



The cytoplasm of the neuroblasts is very pale and finely 

 granular, and their nuclei are pale and refractive, while the 

 daughter cells stain much more deeply throughout and thus 

 resemble the elements of the integumentary ectoderm {E). 

 The polar axes of the neuroblast spindles seem always to 

 be directed at right angles to the surface of the body in 

 embryos of this stage. The " Punktsubstanz " {P) makes its 

 appearance in the bases of the lateral cords, which in the 

 section figured are separated by a pyramidal mass of cells, the 

 median cord. Heading this mass of cells is another neuroblast 

 {M) apparently pushed below the surface by the bulging of the 

 lateral cords. Owing to the shape of the space to which the 

 terminal cell is confined, its daughter cells cannot arrange 

 themselves in a straight column, but lie heaped up somewhat 

 irregularly. While the four lateral neuroblasts represent the 

 cross-section of four continuous longitudinal rows of cells, the 

 median cord neuroblasts are isolated elements which arise 

 intersegmentally, but soon move forward between the two 

 connectives, and finally come to lie just back of the posterior 

 commissure in each segment. At a still later stage each is 

 incorporated together with the mass of cells to which it has 

 given rise in the posterior part of a ganglion. Inasmuch as 

 each postoral ganglion appears to be provided with one of 

 the median cord cells, these elements constitute a ninth un- 

 paired and interrupted row of neuroblasts extending like the 

 lateral rows from the mouth to the anus. The interruptions 

 occur at the points where the commissures are formed and at 

 the intercommissural spaces. I have not yet been able to find 

 median cord neuroblasts in very young embryos, but I do not 

 doubt that they differentiate from the primitive ectoderm at 

 the same time as the neuroblasts of the lateral cords. 



The brain and optic ganglia arise as strings of cells budded 

 off from the sixteen rows in the same manner as the ventral 

 ganglia originate from the eight rows of neuroblasts. 



In a stage succeeding the one figured, the daughter cells (6) 

 of the neuroblasts themselves divide, the axes of their spindles 

 lying at right angles to the axis of the mother spindle. Thus 

 each neuroblast becomes the end of a pillar consisting of from 

 two to four rows of ganglion cells which have arisen by division 

 of the elements in the original single row. 



