MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 318 



XI. Notes on the embkyonic development of the brain and ventral cord. Plates xviii 



AND XIX. 



The ffenei'al morphology of the brain of Liimilus can not be with certainty fully worked out 

 without a knowledge of the embryology of the nervous system, especially of the relations of the 

 neuromeres of the head and of the changes which take place iii the relative arrangement of these 

 nenromeres, and of the lobes formed by them during the development of the parts of the head in 

 the later embryonic stages. 



Also, the mode of development of the ruffle-like masses of chromatic or aprotoplasmic gang- 

 lion cells will have to be studied during the period of larval life succeeding the first molt, and be- 

 fore the body becomes, say, about an inch in length without the caudal spine. With the results 

 of Prof. Patten's work we are not satisfied, as we feel by no means sure that his diagrams in his 

 Figs. 7, 8, 9, and 10, however ingenious, are in all respects reliable, and are confident that n<) more 

 than a single pair of median eyes actually exist in nature. We have found no traces of a second 

 pair of median eyes, nor has Kishiuouye detected them. Until, then. Prof. Patten j)ublishes his 

 observations in full we should prefer to wait before entirely accepting his diagrammatic figures 

 and some of the views expressed in his paper. 



The most important observation on the development of the brain of Limulus are those of Mr. 

 Kishiuouye, who seems to have paid special attention to this subject. He states: 



The lateral halves of the nervous system develop iiulepeiidently of each other. Each half of the brain with its 

 rorrespoiidiiii; ventral nerve cord is produced as a continuous long cord of epiblastic thickening just inside the 

 base of the appendages (Figs. 12-14). It is easy, however, to distinguish the brain fiom the ventral nerve cord. 

 When they are first formed, the former is very much broader than the latter, occupying almost the whole of the 

 segment of the cephalic lobe, while the ventral nerve cord occupies only about one-third of the breadth of e.ach 

 ijppendage-beariug segment (Fig. 12). 



Kishiuouye th<3n describes the "paired small invagination" which appears in the lateral part 

 or margin of the brain, and another paired epiblastic invagination along the anterior internal 

 corner of the margin of the brain. These cephalic invaginations are, however, very shallow, and 

 disappear before the separation of the nervous system from the epiblast takes place. 



portions of these lobes belong to the lobes in question. I had regarded them as lateral lobules of the cerebral lobes, 

 l)ut now accept Viallanes' view as to their nature. 



M. Viallanes has also svicceeded better than myself in making a wax model of the interior structure of the brain, 

 giving a dorsal, ventral, and a sagittal view, and I reproduce his figure of the model of the dorsal view as it shows 

 better than 1 could do the relation of the optic and ocellar lobes to the cerebral lobes. I am able to confirm the 

 accuracy of his work, and am glad to bear tribute to the skill and iiatieuce he has shown in working out the struc- 

 ture of the brain of this animal. On pages 449 and 450 he discusses the organization of the brain of Limulus and 

 of Arachnids. He very briefly (in three lines) states his belief that the two pedunculated bodies of Limulus .are the 

 homologues of the "striated organ," of Arachnids, but does not enter into details, or attempt to show how the two 

 bodies in Limulus can be homologized with the single stratified organ of Arachnids. Neither does he refer to the 

 very great differences between the size and shape of the cerebral hemispheres of Limulus and Arachnids, and to 

 other ditt'erences to which I have drawn attention. I can not agree with M. Viallanes that the brain of Limulus 

 is the exact homologue of the brain of Arachnides; I hold that the " deuto cerebrum" does not form a part of the 

 brain proper j_ that it is not fused with it, that it is post or at least paratesophogeal, since it innervates the first pair 

 of appendages, and this shows that Limulus is in this respect more primitive than the Arachnids. The fusion of 

 what were originally distinct ganglia (see my PI. XVIII, Fig. 7) has never gone on so completely as in the Arachnids. 

 Hince the brain characters, together with the absenc^e of the urinary tubes, and of trachea', and the presence of 

 branchi;p, besides the shape of the six pairs of cephalic appendages and the large median eyes, as well as other 

 minor characters, are sufficient in importance to make Limulus the reiiresentative of a class by itself, with which 

 the trilobites should be associated. 



I may be allowed to add that M. Viallanes' contemptuous critique of my first very imperfect account of the 

 brain of Limulus, published in 1880, is a grain harsh and unfair, since before writing it he must have had in his hands 

 my brief abstract of the present paper published in the Zooiogischer Anzeiger for April 20, 1891, and a copy of which I 

 mailed to him. Even in my first account published in 1880, while I incorrectly stated that the brain was uusymmet- 

 rical, I called attention to and figured the " nucleogenous bodies, " and pointed out their enormous development. To 

 my discovery of these important structures M. Vi.allanes condescends to make no reference; I had also pointed out 

 the differences between the large ganglion cells and the small aprotoplasmic ones, or the "nuclei "composing the 

 nucleogeneous bodies. I did not point out clearly the limits of the median and lateral eye lobes, and those of the 

 cerebral lobes, because in my osmic acid preparations (made in 1879) they are not shown so well as in those stained 

 with caimiue. 



