DEVELOPMENT OF THE SQUID. 13 



which passes gradually into the external yolk sac ; the body-yolk, y" , much smaller than 

 the head-yolk, and separated from it abruptly by a well marked change of outline ; and 

 the little mass of yolk, y"" , at the dorsal end of the body, constricted off from the mass, 

 y"\ by a deep groove. 



Fig. 14, plate 2, represents a view of the posterior surface of a somewhat older embryo, 

 as seen from the left side. 



The mantle is now large and bowl-shaped, and covers the greater part of the body 

 dorsal to the eye-stalks. Chromatophores now begin to make their appearance around 

 the posterior side of the edge of the mantle, and those which first appear are of a dark 

 brown color. 



The gills, g, have lengthened considerably, and are divided by constrictions into a 

 series of enlargements, the dorsal one being much larger than the others, and becoming 

 the branchial heart. The inner and lateral folds of the siphon have completely united 

 with each other, and at the point of union the siphon is also united to the body wall, and 

 the retractor muscle of the siphon, am, now runs back to unite with the inner anterior 

 surface of the mantle. The otocysts have almost met each other upon the median line, 

 under the siphon, and their walls are now very thin. The eye-stalks, es, are most promi- 

 nent at this stage, and soon begin to disappear. 



A comparison of this figure with Grenacher's figure 10, which represents about the 

 same stage of development, will show that certain organs are more accelerated in Loligo 

 than in Grenacher's species, while others are retarded. The yolk sac of Grenacher's 

 embryo has disappeared, while it is as large as ever in Loligo. The mantle is much 

 further developed in Grenacher's embryo, and covers the whole base of the siphon, which 

 is almost entirely uncovered in Loligo, even when the animal is retracted into the mantle. 

 The posterior or siphonal pair of arms, a', are well developed in Loligo, and just makino- 

 their appearance in Grenacher's embryo as buds from the next pair of arms. 



The embryo shown, from the right side, in the next figure, plate 3, fig. 15, has assumed 

 the general form of the adult, and the eye-stalks have almost disappeared, although, as 

 shown in a posterior view, fig. 16, the eyes are very prominent still, and are directed more 

 towards the ventral surface than they are in the adult. 



The mantle now covers about one-half the entire length of the embryo, exclusive 

 of the yolk-sac, and the neck-cartilage, nc, has made its appearance, forming a support for 

 the edge of the mantle, on the middle line of the anterior surface of the head. The 

 posterior surface of the mantle is now pretty well covered with chromatophores, which 

 at this stage possess remarkable power of expansion and contraction, and render 

 the living embryo a very beautiful and wonderful sight under a low magnifying power. 

 They are, as yet, entirely absent from the anterior surface of the mantle. 



About this time small polygonal areolations, much like epithelial cells, begin to 

 make their appearance on the posterior surface of the mantle, and soon spread over 

 the whole mantle, except the middle line of the anterior surface, as shown in the 

 figure. At a later stage, figs. 17 and 18, they cover the head and arms as well as the 

 mantle, and still later they make their appearance upon the surface of the siphon. 



Upon cursoi'y examination, they resemble epithelial cells so much that they might 

 readily be mistaken for them ; but Avhen more carefully examined with a high power. 



