- 12 — 



development 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 develop- 

 ment 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, beginning, 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 tinge deeply with 

 hematoxylin; 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 

 pointing in different directions, the rounder nuclei of the 

 outer layer, which measure '01 mm. in length and '008 mm. 

 in breadth, being mostly inclined at an angle of about 

 75° to the surface; the more oblong nuclei of the inner 

 layer measuring '0i mm. in length and '004 mm. in breadth, 

 lie parallel to the outer surface of the embryo. This 

 inner layer is continued forwards, and forms a skin 

 covering the inner surface of the brain; it is of mesoder- 

 matic origin. 



The first step towards the differentiation of the brain 

 out of the epidermis just described consists of a rapid 

 thickening of the outer or germ layer, through the mul- 

 tiplication of its cells. It soon becomes six to seven cells 

 thick, and the nuclei have no longer any common direc- 

 tion. Through the thickening of the outer layer the 

 surface of the embryo has here become a little arched. 

 After this the outer or germ layer again becomes thinner, 



