440 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



The antennular ganglion is in close union with the optic ganglion and unites also with the 

 antennal ganglion which lies along the sides of the stomod;Buui, extending slightly behind it. 



The histological difierentiation of the nervous elements is not very considerable at this stage. 

 In the ectoblastic thickenings, out of which the nervous system is formed, we can distinguish three 

 kinds of cells: (1) the superficial cells, (2) the central cells, and (3) the accessory cells which come 

 from the yolk. These are best seen in a section of the antennular ganglion (Fig. 116). The outer 

 cells (1), which form the integument, possess very large granular nuclei. Some of these, on either 

 side of the middle line, can be distinguished beyond doubt as the nuclei of those large ganglion 

 cells so characteristic of later stages (see Figs. UG, 147, 169, 170, 191, g. c). They possess a more 

 or less definite cell body of a round or oval contour. In preparations this is fine grained and, like 

 the nucleus, stains but feebly. The weak stain of the nucleus is due to its very line and loose 

 chromatin reticulum. Karyokinetic figures attest to the multiplication of these cells (Fig. 191), and 

 it is highly probable that they give rise to similar cells which occur in both larva and adult. But 

 what is remarkable in the earlier stages is their enormous size and their peripheral position. 

 Keichenbach calls attention to similar cells iu the embryo crayfish, which also arise from the outer 

 layer of ectoderm cells and for a long time help to form the outer wall of the body. It is hardly 

 probable, however, that these cells are relatively more highly differentiated in respect to their 

 ultimate function than any of the surrounding cells which take part in the nervous system. The 

 central cells (2) are the ordinary ganglion cells of the cortex which inclose the fibrous masses. 

 They have smaller and less regular nuclei, which stain very heavily. Cell boundaries are entirely 

 effaced, and the cell protoplasm is reduced to a minimum. The accessory cells (3) rest on the 

 dorsal surface of the thickening, and represent indifferent or wandering cells derived from the 

 yolk (Fig. 116, cts., PI. XLI, mes.). 1 am not prepared to say that any of the ultimate nervous 

 cells are derived from tiiis source, but I am certain that cells migrate from the yolk and attach 

 themselves to the ectoblastic thickening out of which the nervous system is formed, and that they 

 multiply by indirect division. It is probable that the connective tissue sheaths of the nervous 

 system may be due, to some extent at least, to such cells. The ectoblastic thickening is increased 

 by the radial division of superficial cells and by the horizontal division of the deeper cells. 



In the larval and adult stages the large balls of fibrous substance, particularly those of the 

 brain, are surrounded by a delicate cell layer or internal envelope. The nuclei are small and 

 spindle-shaped and form an exceedingly thin sheet. It is ])ossible that this represents intrusive 

 mesoblast, derived from the yolk. Reichenbach states very positively that in Astacus connective 

 tissue cells are squeezed into the fiber balls and eventually surround them. I have no positive 

 evidence to show that the cells in question are not of ectodermic origin, but the behavior of the 

 wandering yolk cells renders it probable that they should rather be referred to the mesoblast of 

 the embryo. 



The Punct-substanz of Leydig or fibrous substance is not present at this time, unless it is 

 represented by a very delicate reticulum in the midst of the nerve cells of the ganglia of the first 

 antennse on their dorsal surface next to the yolk. Degenerating cells (Fig. 114 s') occur in abun- 

 dance close upon the optic ganglia and the ventral ectodermal thickening. 



It may be interesting to notice that the structure of the antennular ganglion (Fig. 116) is 

 similar to that of the optic lobe. In either case there is a peripheral tier of cells possessing large 

 granular nuclei, an inner layer with smaller nuclei, and an imperfect layer of investing cells. 



Passing to Stage vri (PI. XLiv) we find the nervous system still very rudimentary. The super- 

 ficial cells, particulai'ly iu the region of the optic lobes and the antenuiB, have large nuclei, which 

 can be seen iu the act of division, dividing both longitudinally, thus increasing the superficial 

 area of the plate, and also tangentially, in this way adding to its thickness. The very intimate 

 union of the optic ganglion (Fig. 132 G. L.) with the antennular ganglion (S. 0- 0.) is still very 

 noticeable, and the delicate investment of these parts on the side of the yolk [mes.) is more 

 marked. 



Punct substanz has definitely appeared in the supra-oesophageal ganglion where there is a 

 marked transverse commissure, and can even be distinguished in the oesophageal commissures. 

 It forms a very delicate protoplasmic reticulum, and there can be no doubt -that the fibrous sub- 

 stance of this part of the nervous system arises as an outgrowth from the protoplasm of ectoderm 

 cells. 



