THE DEVELOPMENT OF THE SQUID. 



371 



facilitate comparison with the figures which follow, and 

 with the adult animal. The mantle (ni) 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 tins (f) 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'), but 

 the latter have begun to bend around upon the posterior 

 surface of the body. The eye- 

 stalks (es) are now extremely prom- 

 inent 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 appear- 

 ance. 



*Fio. 197. A similar view of an older em- 

 bryo, with its dorsal surface below. (Drawn 

 from nature by W. K. Brooks. ) 



For explanation of letters see Fig. 195. FIG. IDT. 



The three pairs of arms (a) are much elongated, and 

 begin to bend away from the surface of the yolk, which is 

 now divided into three well-marked regions : the external 

 yolk (y 7 ), the portion within the head-region and eye- 



