242 AYERS ON THE DEVELOPMENT 



nuclei have migrated to the dorsal ends of the cells (pi. 23, figs. 9-12), while the walls of 

 their ventral ends have been so compressed as to produce in this manner the peculiar 

 fibrous structure of the ventral portion of the invaginated area. (Compare Hatschek, 23, 

 Taf. ii, fig. 2.) There are differentiated on either side of this central, invaginated portion 

 two limited tracts of cells within the ectodermal layer, which ultimately go to form a 

 part of the nervous system. These two tracts have been designated Seitenstrange by 

 Hatschek. These ^lateral cords" are composed of large cells with scarcely distinguishable 

 cell walls, which enclose large spherical nuclei containing from one to five nucleoli and 

 their systems of radiating nucleolar fibres and granules. The nucleolus is usually angular 

 or bar-shaped and eccentric in position. The nuclei of these ganglionic cells are from 

 three to five times the diameter of the nuclei of the ordinary ectoderm cells. 



The structural conditions to be seen in sections have been described, in the case of the 

 earliest stages of the germinal band, in connection with the account of the origin of the 

 mesoderm. I have not been able to trace the origin of the lateral cords of the nervous 

 system back to such a definite tract of cells as Hatschek has in his studies on Bombyx 

 chrysorrhoea. On the contrary, in sections of the germinal band before the appearance 

 of the median invagination there are to be seen, in the region of the future side cords, two 

 lateral grooves. The cells in these regions have the characters of the cells invaginated for 

 the middle cord and they thus indicate the origin of the two lateral cords as invaginations 

 of the superficial ectoderm, and not as linear tracts of cells budded off from the inner ends 

 of the epithelial cells of the germinal band, as Hatschek has figured and described for 

 Lepidoptera (k>c. cit., pi. I, fig. 6, p. 8). The sections of the stages in which these cells 

 assume their characteristic appearances show them occupying the region of the future 

 lateral cords, but not confined within such definitely marked areas as Hatschek figures. 

 Before the appearance of the appendages the mesoderm is seen to lie as a continuous, 

 thick layer of cells apposed to the germinal band. This condition persists in the abdomi- 

 nal region after the thoracic appendages have grown out, but disappears before the invagi- 

 nation for the nervous system has extended itself into this region. (PI. 22, figs. 27 and 28.) 

 It divides into two lateral plates as the invagination advances. In their outer halves these 

 lateral plates of mesoderm are split into two sheets which contain between them the primi- 

 tive body cavity (pi. 23, fig. 13,1 and figs. 11 and 12, be), but the latter soon disappears by 

 the fusion of its walls. The separation of the splanchnic and somatic layers of the meso- 

 derm occurs before the invagination for the middle cord, but after the formation of the lat- 

 eral cords of the nervous system. The primitive body cavity exists in the form of a pair of 

 tubes extending from the head region backward for a greater or less distance ; but on account 

 of the retardation in the development of the abdominal mesodermic plates, the body cavity 

 of the thoracic region has been changed by the process of segmentation into a number of 

 closed sacs 1 before the cavity has appeared in the abdominal region. The body cavities 

 of the opposite sides do not communicate, although the mesodermic plates have not been 

 divided in the median line. Subsequently the mesoderm occupies the lateral region of 

 the germinal band and is entirely lacking in the region of the middle cord. After the 



1 The posterior wall of each sac is continued backward rior wall of the mesodermic segments of worms. The fate 

 for a short distance in the form of a short pocket, reminding of these pockets is not yet known, 

 one of the enlargements in the splanchnic half of the poste- 



