606 L. H. GOUGH. 



Of sense-organs only the median eyes and the coxnl sense- 

 organs have as yet made their appearance. 



The prseoral part of the brain consists, as in Stage III, 

 of two distinct tracts, distinguishable by size, colour, and 

 uumber of nuclei. The moi'e auterioT part consists of closely 

 packed, small and darkly stained vmclei, averaging '0034 

 mm. in size. Nowhere in the rest of the brain are the nuclei 

 so small, so densely packed, or sr darkly stainable. We 

 shall call this part of the brain the accessory brain. In all 

 other parts of the brain the nuclei • -re less densely packed ; 

 they average '01 mm. in diameter. The brains of adult 

 Admetus also show the same division. 



These two parts of the brain, following Laurie (31), who 

 has briefly referred to the difference, are also said to be 

 distinguishable in Scorpions. 



I am in a position to give more notes about the develop- 

 ment of the ventral ganglion-cord. This last extends on the 

 one hand from the cerebral ganglia, into which it merges 

 imperceptibly, and runs along the ventral side of the cephalo- 

 thorax and ends in the abdomen. In the latter it merges 

 into the ectoderm so continuously that it is impossible to say 

 where the ventral ganglion-cord begins and the ectoderm of 

 the abdomen ends. 



As all the different stages of the embryonic development 

 of the brain are to be found in the ventral ganglion-cord of 

 one embryo, it will be as well to give an account of it, begin- 

 ning, for the sake of simplicity, with the distal and least 

 differentiated part of the cord (see fig. 8). 



If we examine the epidermis covering the median ventral 

 line of the abdomen at the part where it is thinnest, we find 

 that it consists of two layers of cells, surrounded by much 

 protoplasm. The cell walls are not visible. The nuclei are 

 oblong and tiuge deeply with hajmatoxylin ; the chromatin 

 not being equally, distributed, but collected in small masses, 

 gives the nuclei a spotted appearance. The micro-nucleus is 

 conspicuous in several of the nuclei. 



The nuclei of the two layers lie with their axes pointiug in 



