12 ARACHNIDA. 



3. The Formation of the Organs. 

 A. The Nervous System and the Eyes. 



The ventral longitudinal commissures arise early in the form of 

 two thickened bands situated at the sides of the median groove, and 

 extending the whole length of the body (Fig. 7, A and B). A 

 segmentation into the ganglia is soon apparent. The increase in size 

 of these ganglia (according to Kowalevsky and Schulgin) is due to 

 the development of ten to twelve pit- like depressions on each 

 segment, these representing points of specially active cell-growth. 

 Fifteen to twenty such depressions are also found on the cephalic 

 segment. They suggest the vestiges of sensory organs, which 

 finally disappear when the chain of ganglia becomes detached 

 from the ectoderm. The invaginated median strand (the neural 

 groove) also seems to participate in the formation of the ventral 

 chain of ganglia (Patten, Laurie), but this point is not yet estab- 

 lished. We are unable to determine from the statements of the 

 above authors whether the chain of ganglia and the supra-oesophageal 

 ganglion form one continuous rudiment, or whether these two parts 

 of the system arise separately. 



The supra-oesophageal ganglion seems to arise in close relation to 

 the invaginations which were evident in earlier stages, first as rounded, 

 and then as semi-circular depressions on the cephalic lobes (Fig. 4, 

 C, and 7, A and B). While these cephalic pits are still shallow, a 

 distinct thickening of the ectoderm takes place between them (Fig. 

 12, A). This forms the median wall of the two pits. We may 

 assume that this thickening is chiefly concerned in the formation of 

 the brain. Later, the pits become deeper, their apertures become 

 narrower, and shift backwards, as will be further described in 

 connection with the formation of the eyes. It appears that the 

 rudiment of the brain at the same time becomes gradually detached 

 from the pits, and takes up a more lateral position. This accounts 

 for the fact that, at a later stage, the rudiment of the brain lies 

 laterally to the pits. The cephalic pits furnish the rudiments of 

 the median eyes. 



The above description does not altogether agree with statements made by 

 Laurie, Kowalevsky, and Schulgin, who regard the brain as more closely 

 connected with the depressions, indeed, as directly derived from them. We are 

 not able, however, to interpret the figures given by Laurie and Patten in any 

 other sense. There would still be a connection between the formation of the 

 brain and that of the median eyes, but it would not be so close as the authors 

 above-named imply. 



