THR DEVELOPMENT OF ADMETQS PUMILIO. 611 



I have only been able to find one in each neuroraere. This 

 probably corresponds to the larger sense-oro-ans in Patten's 

 figures. It is to be regretted that Patten did not publish 

 any drawing of a transverse section of such a sense-organ, or 

 a description of tlieir development. Kishinouye (22) also 

 states that the nervous system of Limulus consists at one 

 period of peculiar cell-groups resembling ommatidia. Ac- 

 cording to the drawings he gives, he must have cut them at 

 right angles to the direction in which I have cut tkem in the 

 embryo descriljed. 



According to the description just given, we find that the 

 brain is derived in all its parts from the ectoderm, as 

 follows : 



(i) Directly, as regards the germ groups. 



(ii) Indirectly, through the epidermis, as regards the 

 sheaths surrounding the radial supporting-fibres 

 and the tubes. 



In more mature portions of the brain the cell-elements 

 have reached a higher development. We find in the ventral 

 ganglion -cord the two usual kinds of nerve-cells, lying the 

 one amidst the other. 



(i) Cells with smaller, generally oblong nuclei, measuring 

 •01 mm. by "007 mm., without visible protoplasm ; these form 

 the chief part of the brain behind the accessory brain. It is 

 impossible to distinguish these nuclei from those of the 

 germ- layer or from the neuroglia. 



(ii) Cells with larger, generally round nuclei, measuring 

 •015 mm. in diameter; these nuclei do not generally stain so 

 deeply with haematoxylin. They are usually surrounded b}^ 

 protoplasm, and often have very distinct micro-nuclei. They 

 are usually to be found near the commissural substance. 

 They much resemble the ganglion-cells of other animals. 



The real meaning of the just-described process of develop- 

 ment becomes clearer when we compare the development of 

 the central nervous system of the Arachnids with that of 

 other classes of animals. Since the Arachnid brain has 

 reached such a high degree of perfection, we must look for 



