AND EMBRYOLOGY OF LIMULtTS. 37 



Development of the internal organs. Although a good many eggs were sliced, I was 

 unable to discover any in the stage when the ectoderm and endoderm are differentiated, 

 nor to examine the embryo in the gastrula condition, if there be such. The eggs were 

 either in the stage of segmentation of the yolk, or the embryo was so far advanced that 

 the mdications of the segments had appeared. This period of development of the gastrula 

 is evidently intermediate between the stages, plate 3, fig. 7, and fig. 10 of my first memoir 

 The succession in which the more important system of organs arise, is as follows : — 

 first the nervous system ; long afterwards the muscles and the heart. Those organs are 

 well developed before the larva hatches, though the first indications of the mesoderm were 

 not observed. It is not for some time after hatching that the digestive canal as a whole is 

 formed ; although in the gastrula condition an archenteron may probably be developed, 

 I have been unable to detect, after making numerous" sections of eggs and embryos, any 

 traces of the stomach and intestine until long after the larva has hatched. The primitive 

 liver-tubules and the ovax'ies seem to arise at about the same time after the digestive canal 

 is indicated. The development of the renal organs was not tra.ced, no indications of these 

 organs being detected. 



The eyes begin to form at the time of hatching, before the digestive tract is indi- 

 cated. But little attention was devoted to the mode of development of the compound 

 eyes. They are then very small black spots, the rudimentary corneal lenses few 

 in number, and conical. The black retina is underlaid by a white mass ; plate 4, fio-. 4 

 represents one of the ocelli at or soon after exclusion from the egg ; the external region is 

 clear and made uj? of about twenty elongated epithelial cells, with a distinct refractive 

 nucleus and granules ; whether these are pigment cells or not we did not farther observe ; 

 underneath this area is the dark pigment mass in which no cells could be detected with a 



objective and B eyepiece ; the ends of the epithelial cellsseem to sink into the mass. 



Development of the nervous system} After a number of unsuccessful attempts at 

 discovering the first indications of the nervous system, I at length discovered, in thin 

 sections kindly made for me by Prof T. D. Biscoe, the nervous tract in a transverse 

 section of an embryo in an early stage of development, corresponding to that figured on 

 plate 6, fig. 10, of my first memoir. The period at which it was first observable was 

 posterior to the first blastodermic moult, and before the appearance of the rudiments of 

 the six pairs of cephalothoracic lunbs (gnathopods). The primitive band now entirely 

 surrounds the yolk, being much thicker on one side of the egg than on the other, the 

 limbs budding out from this disk-like, thickened portion, most of which represents the 

 ectoderm. At the time the nervous cord was observed it was entirely differentiated and 

 quite distinct from the surrounding tissue of the ectoderm.^ 



At a later stage in the embryo, represented by plate 5, fig. 16, in my first memoir, at a 

 period when the body is divided into a cephalothorax and abdomen, and the limbs are 

 developed, by a series of sections made parallel with the under surface of the body, I could 



1 The principal points in this section were originally ^ Plate 4, fig. 3, represents the nerve cells, and fig. 3a, 



printed in a short notice in the American NaturaUst, July the cells of the mass of connective tissue in which the two 

 1875, IX, 422. cords are embedded, from a freshly hatched larva. 



