20 THE DEVELOPMENT OF 



my observations go, only is seen in stage (7. In the later 

 stages there is nothing to be seen in this region which 

 could be regarded as a derivative from it, unless it be the 

 hinder margin of the carapax, shown in figs. 42. and 54. 



NERVOUS SYSTEM. 



At about the earliest stage at which any rudiments of 

 organs appear we can distinguish roughly the outline of 

 the nervous system. In figure 8 (this Bulletin, Vol. 

 xvm) which represents a surface view of the gastrula stage, 

 there is shown an aggregation of ectodermal cells in front 

 of (below in the figure) the blastopore. Comparison of 

 this with figure 10 of the same plate, and with fig. 1 of 

 my paper on the development of the eye (*87) show that 

 in this aggregation we must recognize, not only the rudi- 

 ments of the thoracic-abdominal area but of the nervous 

 system as well. In fig. ,10 (reproduced at a slightly later 

 stage in fig. 28) we can clearly see the two oval optic lobes 

 and leading from them backwards two cords of cells, the 

 rudiments of the future cords, reinforced, as shown by 

 sections, by a deeper layer of histologically different cells 

 which I interpret as mesodermal bands (see figs. 29-31). 

 These ectodermal nervous rudiments are composed of cells 

 more columnar than those occurring between and the ner- 

 vous system may be described as a pair of longitudinal 

 thickenings which are as yet unconnected in front, and 

 are but a single cell in thickness. 



In stages B and (7, much the same conditions are retained 

 except that the nervous system is several cells thick, but 

 a study of figure 11 makes it evident that we must consider 

 all of the ganglia of the nervous cord, except those con-, 

 nected with the eyes, as primitively belonging to the post- 

 oral series. I may say here that I am inclined to believe 

 that I fell into error in my account of the development of 



