36 A. S. PACKARD, JR., ON THE ANATOMY 



central masses are composed of nerves, with a few nuclei ; the fibres are mostly cut across, 

 but occasionally short bundles of nerve fibres are seen lying across the cut ends of the 

 others, though near the outer edge fibres are seen originating from the cells and passing 

 in to the nerve mass. 



Further Contributions to the Embryology of Limulus. 



The blastodermic skin or serous membrane. In my paper in the Memoirs of the Boston 

 Society of Natural History I stated that the blastodermic skin, just before being moulted, 

 consisted of nucleated cells, and I also traced its homology with the so-called serous 

 membrane or outer " Faltenhulle " of the ectoderm of insects. In 1873,^ by making 

 transverse sections of the egg, I was able to study in a still more satisfactory manner these 

 blastodermic cells, and to observe their nuclei before they became effaced during and 

 after the moulting of the blastoderm. 



On June 17th (the egg having been laid May 27th), the peripheral blastodermic cells 

 began to harden, and the outer layer, that destined to form the outer or " serous " layer, 

 to peel off" from the primitive band beneath. The moult is accomplished by the flattened 

 cells of the blastodermic skin hardening, and peeling off" from those beneath. During this 

 process the cells in this outer layer lose their nviclei, contracting and hardening during the 

 process. Plate 3, fig. 14a shows at o the moulted emj^ty cells with the nuclei empty and 

 beginning to disappear, the walls being ragged and contracted ; at 6 is the layer 

 underneath of lining cells, with granules and distinct nuclei. Figs. 14c and 14cZ show the 

 same cells during the moult, as seen from above and sideways ; 146 represents the normal 

 blastodermic cells, with a large, well-filled nucleus. 



This blastodermic moult is comparable with that of Apus, as I have already observed,^ 

 the cells of the blastodermic skin in that animal being nucleated. This blastodermic skin 

 may also, in its mode of development, be compared with the serous membrane of the 

 scorpion as described by MetschnikofF, and with that of insects, in which at first the blas- 

 todermic cells are nucleated, and appear like those of Limulus. A similar moult takes 

 place in Apus. 



On June 19th, in other eggs, the cells of this membrane were observed to be empty, and 

 the nuclei had lost their fine granules, and were beginning to disappear. The walls of the 

 cells had become ragged through contraction, and in vertical sections short, peripheral, 

 vertical, radiating lines could be perceived. At this time an interesting phenomenon was 

 observed. In certain portions of the serous membrane the cells had become effaced, tran- 

 sitions from the rudiments of cells to those fully formed being seen. In insects and 

 crustaceans, as a rule, the cells all finally disappear, the serous membi'ane being structure- 

 less and homogeneous. The relation of the blastodermic cells in the sei'ous membrane of 

 Limulus is due, without doubt, to the singular function this skin is destined to perform ; 

 i. e., its use as a vicarious chorion, the chorion itself splitting apart and falling off in 

 consequence of the increase in size of the embryo. 



^Tbe substance of this account appeared in the American ■' See Memoirs Bost. Soc. Nat. Hist., ii, 161, foot-note. 



Naturalist, Nov. 1873, vii, p. 675. 



