10 W. K. BROOKS ON THE 



The ridge, a, of figs. 6 and 7, is now divided into three arms, fig. 9, a, upon each 

 side of the body, about half-way between the mantle and the opposite pole of the egg, 

 and therefore much nearer the mantle than at the stage of fig. 6. The yolk is now 

 entirely surrounded by the blastoderm, and has departed still further from the regularly 

 curved shape of fig. 1. The prolongations into the mantle and the eye-stalks are 

 well defined, and the portion of the yolk contained within the body of the embryo, 

 which is not quite half the whole, is separated by a well-marked constriction, just 

 dorsal to the arms from the remainder, which is now nearly spherical. The thin 

 layer of blastoderm which covers this external portion of the yolk is split into two layers, 

 separated from each other by a cavity which is largest along the median plane of the body, 

 and which is traversed by a few branched corpuscles, by the contraction of which, rythmical 

 waves of the outer layer are set in motion on the surfiice of the yolk. The fact that the 

 mantle is dorsal to the row of arms at this period is worthy of notice. 



The next figure, plate 2, fig. 11, is a view of the posterior surfiice of a somewhat older 

 embryo, represented, like the preceding ones, with its dorsal surface above. The mantle, 

 m, now overhangs the body considerably at the sides, as well as posteriorly, and the 

 portion of the yolk which projects into it is more sharply marked off" than before, and 

 is drawn out to a point at the dorsal end. The eye-stalks, es, and their yolk 

 protv;berances, are much more prominent, and the constriction which separates the 

 body from the external yolk is much more marked. The three pairs of arms are a little 

 larger than before, and a cavity is visible in each of them. The inner siphon folds, si, 

 have lengthened, and their outer ends now point towards the outer folds, si', from which, 

 however, they are still widely separated. The most important difi"erences between this 

 and the preceding stage are differences of proportion and relative size, which are 

 sufficiently well shown in the drawings, and do not call for description. 



Plate 2, figi 12, is a posterior view of an older embryo, figured with its dorsal 

 surface below instead of above, in order to facilitate comparison with the figures which 

 follow, and with the adult animal. The mantle, m, has extended its edge sufficiently to 

 form a very well defined mantle-cavity, within which the bases of the gill tentacles, g, are 

 now contained. The tail fins, /', have made their appearance upon the dorsal surface of 

 the mantle, and the rectum, re, is now present as a raised, longitudinal, hollow rod, upon 

 the median line of the posterior surface between the gills. The two inner siphon folds, 

 si, have met upon the middle of the body, and their free edges have bent towards each 

 other to form the opening of the siphon ; but they have not yet united with each other, 

 and the siphon has the characteristics of that of the adult Nautilus. The inner folds are 

 still separated from the outer ones, si, si', but the latter have begun to bend around upon 

 the posterior surface of the body. The eye-stalks, es, are now extremely prominent and 

 conspicuous, and the yolk protuberances no longer entirely fill them, but have begun to 

 decrease in size, thus leaving between the eye and the yolk a space in which the optic 

 ganglion has made its appearance. 



The three pah's of arms, a, are much elongated, and begin to bend away from the 

 surfece of the yolk, which is now divided into three well-marked regions ; the external 

 yolk, y' ; the portion within the head region and eye-stalks, y"; and the portion within the 

 body and mantle, y'". During its development the embryo has undergone an increase in 



