— w — 



inner skin mentioned at the beginning of this description. 

 That the whole of the post-oral ventral ganglion-cord 

 is formed in the manner just described is proved by 

 the presence of numerous remains of tubes, filled with 

 tangled masses of supporting fibre, in the brain-mass. 

 Each such tube denotes the place where there was such 

 a group of cells during the earlier development of the 

 cord. Later on the tubes become entirely obliterated. 

 These groups of cells, with their radial supporting fibres, 

 their sheaths and tubes, possibly represent what Pat- 

 ten (34) compares to ommatidia in his description of the 

 development of the brain of Arthropods. The comparison 

 with sense-organs is certainly very good. I find, howe- 

 ver, more resemblance with the „Hugelorgane der 

 Seitenlinie", as described by Ley dig for fishes, and 

 by the Sarasins (40) for the larvae of Amphibians. 

 Judging by Patten's figures, however, I must conclude 

 that he saw a phenomenon in part different to that which 

 I have just described. In his drawing one sees that each 

 segment of the brain is composed of several sense-organs, 

 with larger ones in the middle line! I have only been 

 able to find one in each neuromere. This probably 

 corresponds to the larger sense-organs 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 their development. Kishinouye (22) also 

 states that the nervous system of Limulus consists at 

 one period of peculiar cell-groups resembling ommatidia. 

 According to the drawings he gives, he must have cut 

 them at right angles to the direction in which I have cut 

 them in the embryo described. 



According to the description just given, we find that 

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

 as follows : 



(1) Directly, as regards the germ groups. 



(2) 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 



